Tổng 2 số là : 428 x 2 = 856

Ta có ; ab +7ab = 856

ab + 700 + ab = 856

2 x ab = 856 - 700

2 x ab = 156

ab = 156 : 2

ab = 78

Vậy 2 số ddos là 78 và 778

#chanh

chị ơi đó phải là ab7 chứ ko phải là 7ab đâu

\(A=\left(4^9\cdot36+64^4\right)\div\left(16^4\cdot100\right)\)

\(A=\left[4^9\cdot4\cdot9+\left(4^3\right)^4\right]\div\left[\left(4^2\right)^4\cdot25\cdot4\right]\)

\(A=\left(4^{10}\cdot9+4^{4\cdot3}\right)\div\left[4^{2\cdot4}\cdot25\cdot4\right]\)

\(A=\left(4^{10}\cdot9+4^{12}\right)\div\left(4^8\cdot25\cdot4\right)\)

\(A=\left(4^{10}\cdot9+4^{10}\cdot4^2\right)\div\left(4^8\cdot25\cdot4\right)\)

\(A=\left(4^{10}\cdot9+4^{10}\cdot16\right)\div\left(4^8\cdot25\cdot4\right)\)

\(A=4^{10}\cdot\left(9+16\right)\div\left(4^8\cdot25\cdot4\right)\)

\(A=4^{10}\cdot25\div\left(4^8\cdot25\cdot4\right)\)

\(A=\frac{4^{10}\cdot25}{4^8\cdot25\cdot4}\)

\(A=\frac{4^2\cdot1}{1\cdot1\cdot4}\)

\(A=4\)

\(B=72^3\cdot54^2\div108^4\)

Ta lần lượt phần tích \(72,54,108\) ra thừa số nguyên tố.

\(72=2^3\cdot3^2\).

\(54=2\cdot3^3\)

\(108=2^2\cdot3^3\)

\(\Rightarrow B=\left(2^3\cdot3^2\right)^3\cdot\left(2\cdot3^3\right)^2\div\left(2^2\cdot3^3\right)^4\)

\(B=\left(2^3\right)^3\cdot\left(3^2\right)^3\cdot2^2\cdot\left(3^3\right)^3\div\left[\left(2^2\right)^4\cdot\left(3^3\right)^4\right]\)

\(B=2^{3\cdot3}\cdot3^{2\cdot3}\cdot2^2\cdot3^{3\cdot2}\div\left(2^{2\cdot4}\cdot3^{3\cdot4}\right)\)

\(B=2^9\cdot3^6\cdot2^2\cdot3^6\div\left(2^8\cdot3^{12}\right)\)

\(B=\left(2^9\cdot2^2\div2^8\right)\cdot\left(3^6\cdot3^6\div3^{12}\right)\)

\(B=2^{9+2-8}\cdot3^{6+6-12}\)

\(B=2^3\cdot1\)

\(B=8\)

Có số HS khối 8 là:200+25=225(HS)

P/s:Đề bài bị thiếu nên sợi chỉ đứt! Không tính dc số HS khối 6,7

\(\frac{1}{2.x}-\frac{1}{1.2}-\frac{1}{2.3}-......-\frac{1}{45.46}=-2\)2

\(\frac{1}{2.x}-\left(\frac{1}{1.2}+\frac{1}{2.3}+.......+\frac{1}{45.46}\right)=-2\)

Đặt \(A=\frac{1}{1.2}+\frac{1}{2.3}+.....+\frac{1}{45.46}\)

\(A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+.....+\frac{1}{45}-\frac{1}{46}\)

\(A=1-\frac{1}{46}=\frac{45}{46}\)

Ta có: \(\frac{1}{2.x}-\frac{45}{46}=-2\)

\(\frac{1}{2.x}=\frac{-47}{46}\)

\(\frac{-47}{-94.x}=\frac{-47}{46}\)

\(\Rightarrow x=\frac{-23}{47}\)

Biết a=b=c=d 

Thay vào M

Ta có: 

\(M=\frac{2a-a}{a+a}+\frac{2a-a}{a+a}+\frac{2a-a}{a+a}+\frac{2a-a}{a+a}\)

\(=4.\frac{2a-a}{a+a}=4.\frac{a}{2a}=4.\frac{1}{2}=2\)

\(\left[\left(3x+1\right)^3\right]^5=15^0\)

\(\Leftrightarrow\left(3x+1\right)^{15}=1\)

\(\Leftrightarrow\left(3x+1\right)^{15}=1^{15}\)

\(\Rightarrow3x+1=1\)

\(\Leftrightarrow3x=1-1\)

\(\Leftrightarrow3x=0\Rightarrow x=0\)

\(\left[(3\times+1)^3\right]^5=15^0\)

\(\Rightarrow\left[(3\times+1)^3\right]^5=1\)

\(\Rightarrow\left[(3\times+1)^3\right]^5=1^5\)

\(\Rightarrow(3\times+1)^3=1\)

\(\Rightarrow(3\times+1)^3=1^3\)

\(\Rightarrow3\times+1=1\)

\(\Rightarrow3\times=1-1\)

\(\Rightarrow3\times=0\)

\(\Rightarrow\times=0\)