Question

bài 7 tự luận nha mn ơi, gấp ạ. 

Answer 1

7:

a: \(3\left(5a+4b\right)-2\left(17a+6b\right)\)

\(=15a+12b-34a-12b=-19a⋮19\)

Ta có: \(3\left(5a+4b\right)-2\left(17a+6b\right)⋮19\)

\(5a+4b⋮19\)

Do đó: \(2\left(17a+6b\right)⋮19\)

=>\(17a+6b⋮19\)

b: \(B=2+2^2+2^3+...+2^{89}+2^{90}\)

\(=\left(2+2^2+2^3+2^4+2^5\right)+...+\left(2^{86}+2^{87}+2^{88}+2^{89}+2^{90}\right)\)

\(=2\left(1+2+2^2+2^3+2^4\right)+2^6\cdot\left(1+2+2^2+2^3+2^4\right)+...+2^{86}\left(1+2+2^2+2^3+2^4\right)\)

\(=31\left(2+2^6+...+2^{86}\right)⋮31\)