Đáp án B

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x:2=y:(-5)

\(\Rightarrow\frac{x}{-5}=\frac{y}{2}\)và x-y=-7

Áp dụng tính chất của dãy tỉ số bằng nhau ta có :

\(\Rightarrow\frac{x}{-5}=\frac{y}{2}=\frac{x-y}{-5-2}=\frac{-7}{-7}=1\)

Suy ra:x=-5.1=-5

         y=2.1=2

Vậy:x=-5;y=2

Tìm x biết:

\(\frac{-12}{13}x-5=\frac{79}{13}\Rightarrow\frac{-12}{13}x=\frac{79}{13}+5\Rightarrow\frac{-12}{13}x=\frac{79}{13}+\frac{65}{13}\Rightarrow\frac{-12}{13}x=\frac{144}{13}\Rightarrow x=\frac{144}{13}:\frac{-12}{13}\Rightarrow x=\frac{144}{13}.\frac{13}{-12}=\frac{12}{1}.\frac{1}{-1}=\frac{12}{-1}=-12\)

tính :

\(\frac{5^4.20^4}{25^5.4^5}=\frac{\left(5.20\right)^4}{\left(25.4\right)^5}=\frac{100^4}{100^5}=\frac{1}{100}\)

so sánh các số: \(2^{150}\)và \(3^{100}\)      

\(2^{150}=\left(2^{50}\right)^{100}\)
\(3^{100}=\left(3\right)^{100}\)\(\Rightarrow2^{50}va3\Rightarrow2^{150}>3^{100}\)

Bài 1; a)Ta có : 2+(-5) = -3

Số đối của các số 2;-5;2+(-5)Lần lượt là:-2;5;3

b)Hai tổng bằng nhau

Bài 2 : 7+(5-13)=-1                                                   7+5+(-13)=-1  

Hai kết quả bằng nhau   

12-(4-6)=14                                                          12-4+6=14

Hai kết quả bằng nhau

Bài 3 : x+4=-2

          x    =-2-4

           x   =-6                                    

a) Ta có: -7x+13>-7y+13

\(\Leftrightarrow-7x>-7y\)

hay x<y

b) Ta có: 11x-1>11y+1

mà 11x+1>11x-1

nên 11x+1>11y+1

\(\Leftrightarrow11x>11y\)

hay x>y

c) Ta có: -19x-37<-19y-37

\(\Leftrightarrow-19x< -19y\)

hay x>y

\(x-y=\left(5+\frac{3}{13}+\frac{7}{26}+\frac{1}{2}\right)-\left(5+\frac{2}{3}+\frac{4}{37}+\frac{5}{111}\right)\)

\(=\left(\frac{1}{2}-\frac{2}{3}\right)+\left(\frac{3}{13}-\frac{4}{37}\right)+\left(\frac{7}{26}-\frac{5}{111}\right)>0\)

=>  x> y