\(\left(x^2-6x+9\right)+15\left(x^2-6x+10\right)=1\)

\(\Leftrightarrow\left(x-3\right)^2+15\left[\left(x-3\right)^2+1\right]=1\)

\(\Leftrightarrow16\left(x-3\right)^2+15=1\)

\(\Leftrightarrow16\left(x-3\right)^2=-14\)

=> Phương trình vô nghiệm 

Gọi (a;b) = d

Khi đó : \(\left\{{}\begin{matrix}a⋮d\\b⋮d\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}a+b⋮d\\b⋮d\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}p⋮d\\b⋮d\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}d=\left\{1;p\right\}\\b⋮d\end{matrix}\right.\left(1\right)\)

Vì \(p\in P;a+b=p\)

nên (a;b) = d < p 

Từ (1) suy ra d = 1 

khi đó (a;b) = 1

Vậy a;b nguyên tố cùng nhau 

P = (a + b + c)³ - 4(a³ + b³ + c³) - 12abc

= (a + b + c)³ - 4(a³ + b³ + c³ + 3abc) 

= (a + b + c)³ - 8c³ - 4(a³ + b³ - c³ + 3abc) 

= (a + b + c)³ - (2c)³ - 4(a³ + b³ - c³ + 3abc) 

Có (a + b + c)³ - (2c)³ 

= (a + b - c)[(a + b + c)² + (a + b + c).2c + 4c²]

= (a + b - c)(a² + b² + c² + 2ab + 2bc + 2ca + 2ac + 2bc + 2c² + 4c²)

= (a + b - c)(a² + b² + 7c² + 4bc + 4ac + 2ba)

Lại có a³ + b³ - c³ + 3abc

 = (a + b)³ - c³ - 3ab(a + b) + 3abc

= (a + b - c)[(a + b)² + (a + b)c + c² - 3ab]

= (a + b - c)(a² + b² + c² + ac + bc - ab) 

Khi đó P = (a + b - c)(a² + b² + 7c² + 4bc + 4ac + 2ba) - 4(a + b - c)(a² + b² + c² + ac + bc - ab) 

= (a + b - c)(-3a² - 3b² + 3c² + 6ba)

= 3(a + b - c)(- a² - b² + 2ab + c²)

= 3(a + b - c)[c² - (a - b)²]

= 3(a + b - c)(a + c - b)(c - a + b) 

Nếu P < 0 thì  3(a + b - c)(a + c - b)(c - a + b)  < 0

<=>  (a + b - c)(a + c - b)(c + b - a) < 0

=> Có ít nhất một hạng tử trái dấu với 2 hạng tử còn lại

Với a,b,c > 0

Giả sử \(\left\{{}\begin{matrix}a+b-c< 0\\a+c-b>0\\b+c-a>0\end{matrix}\right.\) => a;b;c không là 3 cạnh tam giác 

hoặc \(\left\{{}\begin{matrix}a+b-c>0\\b+c-a< 0\\a+c-b< 0\end{matrix}\right.\) cũng tương tự

Vậy a,b,c không là 3 cạnh tam giác 

ĐKXĐ : a;b;c  \(\ne0\)

Ta có : \(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}=\dfrac{1}{2000}\)

\(\Leftrightarrow\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}=\dfrac{1}{a+b+c}\)

\(\Leftrightarrow\dfrac{1}{b}+\dfrac{1}{c}=\dfrac{1}{a+b+c}-\dfrac{1}{a}\)

\(\Leftrightarrow\dfrac{b+c}{bc}=\dfrac{-\left(b+c\right)}{a\left(a+b+c\right)}\)

\(\Leftrightarrow\left(b+c\right)\left(\dfrac{1}{bc}+\dfrac{1}{a\left(a+b+c\right)}\right)=0\)

\(\Leftrightarrow\left(b+c\right).\dfrac{a\left(a+b+c\right)+bc}{abc\left(a+b+c\right)}=0\)

\(\Leftrightarrow\left(b+c\right).\dfrac{a^2+ab+ac+bc}{abc\left(a+b+c\right)}=0\)

\(\Leftrightarrow\dfrac{\left(b+c\right)\left(a+b\right)\left(a+c\right)}{abc\left(a+b+c\right)}=0\)

\(\Leftrightarrow\left[{}\begin{matrix}b+c=0\\a+b=0\\a+c=0\end{matrix}\right.\left(1\right)\)

Từ (1) kết hợp a + b + c = 2000 ta được điều phải chứng minh

b) ĐKXĐ : \(x\ne\pm1\)

\(P=\dfrac{x}{x-1}+\dfrac{3}{x+1}-\dfrac{6x-4}{x^2-1}\)

\(=\dfrac{x\left(x+1\right)+3\left(x-1\right)-\left(6x-4\right)}{\left(x-1\right)\left(x+1\right)}\)

\(=\dfrac{x^2-2x+1}{\left(x-1\right)\left(x+1\right)}=\dfrac{\left(x-1\right)^2}{\left(x-1\right)\left(x+1\right)}=\dfrac{x-1}{x+1}\)

c) ĐKXĐ : \(\left\{{}\begin{matrix}x>0\\x\ne1\end{matrix}\right.\)

\(A=\dfrac{1}{x+\sqrt{x}}+\dfrac{2\sqrt{x}}{x-1}-\dfrac{1}{x-\sqrt{x}}\)

\(=\dfrac{1}{\sqrt{x}\left(\sqrt{x}+1\right)}+\dfrac{2\sqrt{x}}{\left(\sqrt{x}-1\right)\left(\sqrt{x}+1\right)}-\dfrac{1}{\sqrt{x}\left(\sqrt{x}-1\right)}\)

\(=\dfrac{\sqrt{x}-1+2x-\left(\sqrt{x}+1\right)}{\sqrt{x}\left(\sqrt{x}-1\right)\left(\sqrt{x}+1\right)}\)

\(=\dfrac{2\left(x-1\right)}{\sqrt{x}\left(\sqrt{x}-1\right)\left(\sqrt{x}+1\right)}=\dfrac{2\left(x-1\right)}{\sqrt{x}\left(x-1\right)}=\dfrac{2}{\sqrt{x}}\)

a) ĐKXĐ : \(x\ge0;x\ne16\)

\(B=\left(\dfrac{\sqrt{x}}{\sqrt{x}+4}+\dfrac{4}{\sqrt{x-4}}\right):\dfrac{x+16}{\sqrt{x}+2}\)

\(=\dfrac{\sqrt{x}\left(\sqrt{x}-4\right)+4\left(\sqrt{x}+4\right)}{x-16}:\dfrac{x+16}{\sqrt{x}+2}\)

\(=\dfrac{x+16}{x-16}:\dfrac{x+16}{\sqrt{x}+2}=\dfrac{\sqrt{x}+2}{x-16}\)

Gọi số học sinh là x 

Chia cho mỗi cháu 3 cái thừa 5 cái 

=> Số kẹo của cô là \(3x+5\)

Chia mỗi cháu 4 cái thì thiếu 3 cháu 

=> Số kẹo của cô là \(4\left(x-3\right)=4x-12\)

Khi đó 3x + 5 = 4x - 12

<=> 4x - 3x = 12 + 5

<=> x  = 17

nên số kẹo là 3x + 5 = 3.17 + 5 = 56

Vậy cô có 56 cái kẹo 

ĐKXĐ : \(\left\{{}\begin{matrix}4x^2+2y+2\ge0\\3x+y\ge0\end{matrix}\right.\)

Ta có : \(\left(\sqrt{4x^2+3}-2x\right)\left(\sqrt{y^2-2y+4}-y+1\right)=3\)

\(\Leftrightarrow\dfrac{3}{\sqrt{4x^2+3}+2x}.\dfrac{3}{\sqrt{y^2-2y+4}+y-1}=3\)

\(\Leftrightarrow\left(\sqrt{4x^2+3}+2x\right)\left(\sqrt{y^2-2y+4}+y-1\right)=3\)

\(\Rightarrow\left(\sqrt{4x^2+3}+2x\right)\left(\sqrt{y^2-2y+4}+y-1\right)=\left(\sqrt{4x^2+3}-2x\right)\left(\sqrt{y^2-2y+4}-y+1\right)\)

\(\Leftrightarrow2x\sqrt{y^2-2y+4}+\left(y-1\right).\sqrt{4x^2+3}=0\)

\(\Leftrightarrow2x\sqrt{y^2-2y+4}=\left(1-y\right).\sqrt{4x^2+3}\)

\(\Leftrightarrow\left\{{}\begin{matrix}4x^2.\left(y^2-2y+4\right)=\left(y^2-2y+1\right).\left(4x^2+3\right)\\2x.\left(1-y\right)\ge0\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}4x^2=y^2-2y+1\\2x\left(1-y\right)\ge0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}\left[{}\begin{matrix}2x=y-1\\2x=1-y\end{matrix}\right.\\2x\left(1-y\right)\ge0\end{matrix}\right.\)

Với 2x = 1 - y

Khi đó ta có \(\sqrt{4x^2+2y+2}-\sqrt{3x+y}=2x+1\)

\(\Leftrightarrow\sqrt{4x^2-4x+4}-\sqrt{x+1}=2x+1\)      (ĐK : \(x\ge-1\))

\(\Leftrightarrow2\sqrt{x^2-x+1}-\sqrt{x+1}=2x+1\)

\(\Leftrightarrow2\left(\sqrt{x^2-x+1}-1\right)=2x+\sqrt{x+1}-1\)

\(\Leftrightarrow\dfrac{2x\left(x-1\right)}{\sqrt{x^2-x+1}+1}=2x+\dfrac{x}{\sqrt{x+1}+1}\)

\(\Leftrightarrow\left[{}\begin{matrix}x=0\\\dfrac{2x-2}{\sqrt{x^2-x+1}}=2+\dfrac{1}{\sqrt{x+1}+1}\left(1\right)\end{matrix}\right.\)

Phương trình (1) 

<=> \(\dfrac{2\left(x+1\right)}{\sqrt{x^2-x+1}}=2+\dfrac{1}{\sqrt{x+1}+1}+\dfrac{4}{\sqrt{x^2-x+1}}\)

Xét vế trái : \(\dfrac{2\left(x+1\right)}{\sqrt{x^2-x+1}}=\sqrt{\dfrac{4x^2+4x+1}{x^2-x+1}}=\sqrt{\dfrac{5x^2-5x+5-x^2+9x-4}{x^2-x+1}}\)

\(=\sqrt{5-\dfrac{x^2-9x+4}{x^2-x+1}}< \sqrt{5}\) (2) 

Lại có \(2+\dfrac{1}{\sqrt{x+1}+1}+\dfrac{4}{\sqrt{x^2-x+1}}\)

\(=2+\dfrac{1}{\sqrt{x+1}+1}+\dfrac{1}{\sqrt{x^2-x+1}}+\dfrac{1}{\sqrt{x^2-x+1}}+\dfrac{1}{\sqrt{x^2-x+1}}+\dfrac{1}{\sqrt{x^2-x+1}}\)

\(\ge2+\dfrac{\left(1+1+1+1+1\right)^2}{\sqrt{x+1}+1+4\sqrt{x^2-x+1}}=2+\dfrac{25}{\sqrt{x+1}+1+4\sqrt{x^2-x+1}}\)

Dấu "=" khi \(\dfrac{1}{\sqrt{x+1}+1}=\dfrac{1}{\sqrt{x^2-x+1}}\Leftrightarrow\left[{}\begin{matrix}x\approx3,498374325\\x\approx-0,7385661113\end{matrix}\right.\)

Khi đó \(VP\ge3,6\) (3) 

Từ (3) và (2) => (1) vô nghiệm 

Vậy x = 0 => y = 1

Với 2x = y - 1 kết hợp điều kiện 2x(1 - y) \(\ge0\)

ta được x = 0 ; y = 1 

Vậy (x ; y) = (0;1) 

Để 62x1y \(⋮65\)

=> \(\left\{{}\begin{matrix}\overline{62x1y}⋮5\\\overline{62x1y}⋮13\end{matrix}\right.\)

mà \(\overline{62x1y}⋮5\Leftrightarrow\left[{}\begin{matrix}y=0\\y=5\end{matrix}\right.\)

Khi y = 5 thì số đó trở thành \(\overline{62x15}\)

Khi đó \(\overline{62x15}=62000+x.100+15=62015+100x\)

\(=13.4770+100x+5\)

Khi đó  \(\overline{62x15}⋮13\Leftrightarrow100x+5⋮13\)

Với \(x\inℕ;x< 10\)

\(\Rightarrow∄x:100x+5⋮13\)

Tương tự khi y = 0

Ta được \(\overline{62x10}=62010+100x=4770.13+100x\)

Khi đó  \(\overline{62x15}⋮13\Leftrightarrow100x⋮13\)

Với \(x\inℕ;x< 10\)

\(\Rightarrow x=0\) thỏa mãn 

Vậy (x;y) = (0;0) 

 37. We cannot swim in this part of the river because the water is highly polluted. 

38. Mr. Minh has been admired since he dedicated all his life to protecting environment.

39. Lan was sick so she didn't go to school 

??? 40.A lot of Viet Namese people have the custom to buy sugar cane on New Year's Eve