C. answer

\(a,\dfrac{3}{7}=\dfrac{2x+1}{3x+5}\\\Rightarrow3x+15=14x+7\\ \Rightarrow3x-14x=7-15 \\ \Rightarrow-11x=-8\\ \Rightarrow x=\dfrac{8}{11}.\\ b,\dfrac{x+1}{x-2}=\dfrac{3}{4}\\ \Rightarrow4x+4=3x-6\\ \Rightarrow4x-3x=-6-4\\ \Rightarrow x=-10.\\ c,\dfrac{2x+3}{7}=\dfrac{4x-1}{15}\\ \Rightarrow30x+45=28x-7\\\Rightarrow30x-28x=-7-45\\ \Rightarrow2x=-52\\ \Rightarrow x=-26.\)

\(d,\dfrac{6x-5}{-7}=\dfrac{5x-3}{-5}\\ \Rightarrow-30x+25=-35x+21\\ \Rightarrow-30x+35x=21-25\\ \Rightarrow5x=-4\\ \Rightarrow x=-\dfrac{4}{5}.\)

Sửa lỗi: improves

You know, we have many kinds of game that we can play them anytime.But in my class, we find chess the most exciting.Because it improve our brain.

Of course, i think i want to play a game for fun. Because it helps me to relax and good for my happiness.

=> What is your favourite food?

Sửa bài:

   Đặt biểu thức là A. ta được:

     \(\dfrac{A}{5}=\dfrac{5}{1.6}+\dfrac{5}{6.11}+...+\dfrac{5}{26.31}\\ \dfrac{A}{5}=1-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{11}+...+\dfrac{1}{26}-\dfrac{1}{31}\\ \dfrac{A}{5}=\dfrac{30}{31}\\ A=\dfrac{150}{31}.\)

\(\dfrac{5^2}{1.6}+\dfrac{5^2}{6.11}+...+\dfrac{5^2}{26.31}\\ =5^2\left(\dfrac{1}{1.6}+\dfrac{1}{6.11}+...+\dfrac{1}{26.31}\right)\\ =5^2.\left(1-\dfrac{1}{6}+\dfrac{1}{6}+...+\dfrac{1}{26}-\dfrac{1}{31}\right)\\ =5^2.\left(1-\dfrac{1}{31}\right)\\ =5^2.\dfrac{30}{31}\\ =25.\dfrac{30}{31}\\ =\dfrac{750}{31}.\)

\(3\left(x-2\right)=-100+211\\ 3\left(x-2\right)=111\\ x-2=37\\ x=39.\)

\(y:84=\dfrac{106}{84}\\ y=\dfrac{106}{84}\times84\\ y=106.\)