\(\frac{2}{x-1}+\frac{5}{x+2}=\frac{13}{x^2+x-2}.\)

\(\Leftrightarrow\frac{2\left(x+2\right)}{\left(x-1\right)\left(x+2\right)}+\frac{5\left(x-1\right)}{\left(x+2\right)\left(x-1\right)}=\frac{13}{x^2+x-2}\)

\(\Leftrightarrow\frac{2x+4}{x^2+x-2}+\frac{5x-5}{x^2+x-2}=\frac{13}{x^2+x-2}\)

\(\Leftrightarrow\frac{7x-1}{x^2+x-2}=\frac{13}{x^2+x-2}\)

\(\Leftrightarrow7x-1=13\)

\(\Leftrightarrow7x=14\)

\(\Leftrightarrow x=2\)

\(\frac{2x-1}{x-3}=\frac{6x-1}{3x+2}\)

\(\Leftrightarrow\left(2x-1\right)\left(3x+2\right)=\left(x-3\right)\left(6x-1\right)\)

\(\Leftrightarrow6x^2+4x-3x-2=6x^2-x-18x+3\)

\(\Leftrightarrow4x-3x+x+18x=3+2\)

\(\Leftrightarrow20x=5\)

\(\Leftrightarrow x=\frac{1}{4}\)

òm, hay ó^^

Với động từ tobe 

khẳng định S+was/were+0

phủ định S +was/were+not+0 

Câu hỏi was/were+s+0

Với động từ thường

khẳng định S+v-ed/p2+0

phủ định S+did not+v +0

câu hỏi did+s+v+0

Dấu hiệu nhận biết : 

China said that there was a heavy loss of life in the earthquake which struck Tangshan city yesterday. Survivors said that Tangshan, an industrial city of one million people 160 kilometers east of Peking, was completely destroyed. Observers living in Beijing said it appeared that only a small part of the one million inhabitants escaped death or injury. 
Many of the men of Tangshan were working in the mines deep under the earth's surface when the earthquake occurred. Unfortunately, few of these miners have survived. A lot of people were also working in the city's offices. Regrettably, most were killed under falling concrete when the building collapsed. The Chinese authorities have not yet given any information about the actual number of casualties, but it is thought that tens of thousands of people have been killed. Hardly a building has been left standing.

China said today that there was heavy loss of life in the earthquake which struck Tangshan city yesterday. Survivors said that Tangshan, an industrial city of one million people 160 km east of Beijing, was   completely       (51) destroyed. Observers living in Beijing said it appeared that only a small part of the one million inhabbitants (52)      escaped      death or injury. Many of the men of Tangshan were (53)        working         in the mines deep under the earthquake(54)        occured        Unfortunately, few of these miners(55)   have            survived. A lot of people were also working in the city's(56)     offices           . Regrettably, most were(57)       killed         under falling concrete when the building(58)         collapsed      . The Chinese authorities have not yet given any information about the actual(59)    number           of casualties, but it is thought that tens of thousands of people have been killed. Hardly a house has been left (60)     standing       .

People said that he had gone to abroad

\(\Rightarrow\) He was said to go to abroad

Would you mind not using that red pen?

\(\Rightarrow\) I' rather you...you not to use that red pen....

Jack's Parents made him study for this exams 

\(\Rightarrow\) Jack was made to study for his exam by his parents 

People believe paper was invented in China

----> Paper is believed was invented in China

...

 I.water pollution

Artificial causes from people and production affect the water environment

1. From human beings

Every day there is a large amount of domes waste into the environment without treatment, in addition to increasing population, the amount of domes waste also increases. In developed countries, the population growth rate is about 5% while the population growth rate in developing countries is over 2%.

Domes wastewater is waste water generated from households, hospitals, hotels and schools, containing wastes during human daily life and sanitation. The basic composition of domes wastewater is organic biodegradable organic substances (carbohydrates, proteins, oils, nutrients, solids. Depending on living standards and lifestyle, the amount of wastewater as well as the load The substances in each person's wastewater in a day are different, and the higher the living standard, the higher the amount of waste water and the discharge load, so the disease has conditions to spread and pollute the environment.

2. From agricultural production

Agricultural activities such as animal husbandry, animal urine, unprocessed and environmental uneaten food and other agricultural activities. Pesides, fertilizers from rice fields, melons, gardens, vegetables contain toxic chemicals, can contaminate groundwater and surface water.

In the process of agricultural production, most farmers use pesides three times the recommended dose. Not only that, farmers also use banned pesides such as Thiodol, Monitor ... In the process of fertilizing, spraying, farmers do not equip with labor protection tools.

Currently, the use of chemical fertilizers and chemical pesides is rampant in agriculture, causing water sources to be affected. Quantification of residues will sink into the aquifers that affect the amount of water.

Most farmers do not have storage facilities for medicines and medicines when they have not been used and are stored everywhere, including near cafeterias and drinking water sources. Most of the peside bottles used after being discarded were immediately thrown into the fields, the rest were collected for sale.

3. From industrial production

Waste and wastewater, from industrial production activities due to the speed of urbanization, industrialization and development have led to the establishment of industrial parks, thus the amount of waste and wastewater of activities The industry is growing and has not been thoroughly treated directly into the environment or rivers, water sources affect the current water quality.

Natural causes affect the water environment

Any water quality degradation phenomenon causes water pollution. Water pollution due to rain, snow, flood ... or by the casca of living organisms' activity not to mention their corpses. Trees and organisms die, they are decomposed by microorganisms into organic matter. A part will infiltrate into the ground, then penetrate deep into the groundwater, pollute, or follow the flow of groundwater into the big stream.

Flooding can cause water to lose its purity, stir up contaminants, grime in sewers, carry many toxic waste from garbage dumps, and carry on with previously stored substances. .

Prolonged flooding can be more heavily polluted by chemicals used in agriculture, industrial waste, chemical pollution ... Water pollution due to natural factors (volcanoes, storms, floods, erosion ...) may be very serious.

The deterioration of water quality may be due to geological characteriss of water sources such as water on acid sulphate soils often contain a lot of iron, aluminum, water taken from the ground often contains a lot of calcium ...

Measures to overcome the causes of water pollution

People should become more aware of their living environment protection, enterprises should have tanks to treat waste water before being discharged into the environment. The authorities and mass organizations need to regularly inspect companies to avoid the situation of profit that companies do not comply with environmental protection laws.

In addition, people should also protect their family's health by a family water well filter system, a water treatment machine to completely remove contaminants, toxic substances and heavy metals. .. create pure water to drink water, directly without cooking.

The above are the causes of water pollution today, hope that through this arle, people can be more aware to protect the water environment more clean and safe. Because clean water is essential to a healthy life.

II. Air pollution

Causes of air pollution.

Air pollution due to natural factors:

Due to natural phenomena: volcanoes, forest fires, dust storms, or decomposition processes, decaying keratosis - natural plants ... This is an objective cause so it is difficult to predict and prevent.

Air pollution due to human factors

+ Industry: This is the largest source of pollution caused by humans. The process of pollution is the burning of fossil fuels such as coal, oil and gas, creating toxic gases (CO2, CO, SO2, NOx, unburnt organic substances: soot, dust ). Industrial sources are highly toxic, concentrated in a small space, and depending on the scale of production and the fuel used, the amount of toxic and toxic substances will be different.
+ Transportation: This is also a major source of pollution to the air, especially in urban areas and densely populated areas. The process of burning engine fuel creates toxic gases that affect the air such as CO2, CO, SO2, NOx, Pb, CH4 ...
+ Activities: Mainly due to cooking activities that use fuels to create harmful gases that pollute the local households and surrounding households.

Consequences of air pollution

For animals - plants.

+ Air pollution causes harmful effects to all organisms.

+ Sulfur dioxide, nitrogen dioxide, ozone, fluorine, lead ... directly harm plants when entering stomata, damaging the system to reduce drainage and reduce disease resistance.

 + Most fruit trees are very sensitive to HF. When exposed to a concentration of HF greater than 0.002 mg / m3, the leaves are burnt spots, deciduous.

+ Earth's warming due to the greenhouse effect also causes changes in plants and animals on Earth.
+ Acid rain also indirectly affects plants and lacks food Ca and kills soil microorganisms. It causes Al ion to be released into the water to damage the roots (absorbing feathers) and reduce the absorption of food and water.

+ For animals, especially pets, fluoride causes more disasters. They are poisoned by direct inhalation and through the food chain.

+ Acidic air pollutants will combine with water droplets in the cloud to make water acidic. When the water drops on the ground, it will harm the environment: killing trees, animals, fish, etc. Acidic water also changes the nature of water in rivers, streams, etc. harming the births. living things under water.

For man.
Dust:

+ Harmful effects of dust depend on the nature (composition) of dust, dust concentration, dust parle size, contact time and personal response.

+ Dust into the lungs causes mechanical irritation, pulmonary fibrosis leading to respiratory diseases.

+ Dust can cause eye, skin and blood sugar diseases and other body systems (Dust enters the body and dissolves in blood and body fluids), cardiovascular disease ...

+ Dust can cause cancer: dust containing toxic ingredients, asbestos dust ...

Dioxide Sulfur (SO2) and Nitrogen Dioxide (NO2):
Sulfur Dioxide (SO2).

+ Sulfur Dioxide is a gas formed by oxidation of sulfur (S) when burning fuels such as coal, oil, products of oil, sulfide ore, etc. SO2 is a gas that causes strong respiratory irritation, when inhaled. breathing of SO2 gas (even at low concentrations) can cause spasm of the bronchial straight muscles. Large concentrations of SO2 can cause increased mucus secretion in the upper respiratory tract mucosa and bronchial air branches. SO2 affects the function of the lungs, causing pneumonia, chronic bronchitis, causing cardiovascular disease, hypersensitivity in people with asthma, ...

+ SO2 poisoning through the skin reduces alkaline reserves in the blood, eliminates ammonia to the urine and releases saliva.

+ General toxicity of SO2 expressed in disorders of protein and sugar metabolism, lack of vitamins B and C, inhibition of oxidase enzymes.

Nitrogen Dioxide (NO2):

+ Nitrogen Dioxide (NO2): is a brown gas, produced by Nitrogen oxidation at high temperature. NO2 is a dangerous gas, has a strong effect on respiratory organs especially in susceptible groups such as children, the elderly, asthma sufferers. –If contact with NO2 will damage the lung mucosa, increase the risk of respiratory diseases, damage of lung, eye, nose, and throat functions, etc.

Carbon monoxide (CO)

+ Carbon monoxide (CO) combined with hemoglobin (Hb) in the blood to be a stable compound, cacboxy hemoglobin (HbCO), reduces blood oxygen transport capacity, leading to lack of oxygen in the blood ....

Ammonia (NH3)

+ NH3 is a toxic gas that can strongly stimulate the nose, mouth and respiratory system.

+ Exposure to NH3 at a concentration of 100mg / m3 in a short period of time will not have long-term consequences, but if exposed to NH3 at a concentration of 1500-2000 mg / m3 for 30 minutes, it will be dangerous to live.

Hydrogen sulfide (H2S).

+ H2S enters the body through pphooir will be oxidized to sulfate. Low toxicity compounds will not accumulate in the body. About 6% of the absorbed gas will be excreted through exhaled air, the rest after the metabolism is excreted in the urine.

+ At low concentrations, v stimulates on eyes and respiratory tract.

+ Breathing large amounts of H2S mixture will cause a sudden lack of oxygen, which can lead to death from suffocation.

+ Signs of acute poisoning: 

vomiting, gastrointestinal disorders, diarrhea, dry and foul-smelling nasopharynx, eyelid- eyes, ocular conjunctivitis, purulent secretion and vision loss.

+ Regular exposure to H2S at levels below the level of acute toxicity can cause chronic poisoning. Symptoms can be: weakness, nervous system disorders, digestive system, insomnia, chronic bronchitis, ...

Volatile organic compounds (VOCs)

+ Volatile organic compounds (VOCs) include many organic chemicals in which the most important are benzene, toluene, xylene, .. VOCs can cause acute toxicity if exposed at high doses, causing inflammation of the respiratory tract. absorption, gastrointestinal disorders, neurological disorders, hematological disorders, causing liver - kidney damage, causing skin irritation, ... and is the cause of myelosuppression, blood cancer.

Lead (Pb):

Lead (Pb): exhaust smoke from the engine of vehicles involved in traffic contains a certain amount of lead. In addition, lead can be generated from ore mines, from factories producing batteries, synthe plass, paints, chemicals, etc. Lead enters the body through respiratory, food, drinking water, and through skin, through breast milk, etc. Lead will accumulate in the bones and red blood cells causing bone marrow disorders, joint pain, nephritis, hypertension, cerebral complications, poisoning of the central and peripheral nervous system, Red cell rupture causes anemia, causing kidney dysfunction. Pregnant women and children are very susceptible to the effects of lead (causing miscarriage or death, reducing intelligence, ...).

Radon gas.

Radon gas is produced by decaying uranium in nature, which is a heavy gas so it usually exists in the air layer close to the ground. In nature, radon is found in rocky soil, coal slag, construction material dump, in mud. Radon can cling to small dust parles, penetrate the body through the respiratory tract or penetrate the skin, through open wounds that cause lung cancer, leukemia, etc.

For properties

+ Making metal rust.

+ Corrosion to concrete.

+ Abrasion, decomposition of the product surface.

+ Causing discoloration and damage to paintings.

+ Reducing plas strength and discolouration of fabric fibers.

+ Reduce durability of paper, rubber, tanning.

For global

+ Acid rain

+ Greenhouse effect

+ Ozone layer depletion

+ Temperature change.

Remedies

Technical measures:

+ Replace outdated machines, outdated technology lines, causing much pollution by modern, less polluting technology chains.

+ Replace burning fuel from coal and fuel oil by using electricity to prevent air pollution by burnt and SO2.

Planning measures:

+ Minimizing the construction of industrial parks in export processing zones in the city, only retaining enterprises directly serving people's daily needs.

+ Encourage people to travel by public means to minimize congestion and traffic means, thereby reducing the density of dust and waste caused by the combustion of fuel and oil in the air. , especially during peak hours.

+ Create large areas of green trees in the city, set up strips of green trees connecting different areas of the city, especially in areas and streets with many means of passing and happening. congestion.

Some vehicles overcome air pollution

Air purification by biological method

Biofiltration is a relatively new treatment for pollution, which is an attractive method for handling bad odor gases and low concentrations of volatile compounds.

The common shape of a biological filtration system is a big box, some systems can be as big as a basketball court, some systems can be as small as a yard yard (0.76 m3). The main principle of the treatment system is to allow bacteria to come into contact with pollutants in the exhaust gas. This exhaust filter system is a place to store filter materials and breeding grounds for microorganisms. In this system, microorganisms will form a biofilm, which is a thin, moist membrane surrounding filter materials.

During the filtration process, the exhaust gas is pumped slowly through the filtration system, the pollutants in the exhaust gas are absorbed by the filter materials. The polluting gases will be adsorbed by biofilms, where microorganisms will decompose them to create energy and by-products of CO2 and H2O by the following equation:

Organic pollutants + O2 CO2 + H2O + heat + biomass.

Waste gas treatment by biotechnology

Biofilter technology (biofilter) is an emissions treatment method with low investment cost, low cost and environmentally friendly operation, it is a suitable method for handling odors and gases. Volatile organic compounds with low concentrations such as animal feed and cassava starch factories

Air purifier

Based on the principle that charged parles will attract each other, the machine uses technology to emit negative charges into the air, neutralizing the symmetrical charges as harmful positive ions in the environment and creating effects to attract dirt, bacteria, mold ... These negative ions also adhere to toxic parles in the air and positively charged electrosta membrane in the machine will attract and hold these parles.

In closed rooms where, 

Many equipment such as computers, printers, televisions, neon ... are devices that emit many positive ions causing harm to the body, air purifiers will provide negative ions and air conditioners.
Gauze mask:
Activated carbon masks and deodorants have the following effects:

+ Preventing entry into the respiratory tract of microscopic dust parles and other pollution wastes.

+ Filter odor, chemicals, gasoline smell, photochemical smoke, black smoke, pollen. Prevent dust, toxic gases such as CO, SO2, NO2, H2S, NH3 ....

+ Protecting respiratory system, limiting allergic rhinitis caused by air pollutants.

C.themselves

V-ing và to V

Có 2 TH 

nếu danh từ thì sau prefer không thêm gì 

nếu động từ thì sau prefer +Ving