Question

Chung minh rang a thuoc Z thi a (a+2) - a(a-5)-7 la boi cua 7

Answer 1

\(a\left(a+2\right)-a\left(a-5\right)-7=a^2+2a-a^2+5a-7\)

\(=\left(a^2-a^2\right)+\left(2a+5a\right)-7\)

\(=7a-7\)

\(=7\left(a-1\right)⋮7\) (đpcm)

Answer 2

\(a\left(a+2\right)-a\left(a-5\right)-7=a^2+2a-a^2-5a-7\)

\(=\) \(\left(a^2-a^2\right)-\left(2a+5a\right)-7\)

\(=7a-7\)

\(=7\left(a-1\right)\)\(⋮\)\(7\)( đpcm) 

Answer 3

\(a\left(a+2\right)-a\left(a-5\right)-7=a^2+2a-a^2-5a-7\)

\(=\) \(\left(a^2-a^2\right)-\left(2a+5a\right)-7\)

\(=7a-7\)

\(=7\left(a-1\right)\)\(⋮\)\(7\)( đpcm) 

Answer 4

\(a\left(a+2\right)-a\left(a-5\right)-7=a^2+2a-a^2-5a-7\)

\(=\) \(\left(a^2-a^2\right)-\left(2a+5a\right)-7\)

\(=7a-7\)

\(=7\left(a-1\right)\)\(⋮\)\(7\)( đpcm) 

Answer 5

\(a\left(a+2\right)-a\left(a-5\right)-7=a^2+2a-a^2-5a-7\)

\(=\) \(\left(a^2-a^2\right)-\left(2a+5a\right)-7\)

\(=7a-7\)

\(=7\left(a-1\right)\)\(⋮\)\(7\)( đpcm)