Question

2+4+6+...+2n=870

Answer 1

2+4+6+...+2n = 870

<=> 2(1+2+3+...+n)=870

<=> 1+2+3+...+n = 870:2 = 435

<=>\(\frac{n\left(n+1\right)}{2}=435\)

<=> n(n+1) = 435.2 = 870

=> n =29

Answer 2

ko có số 2n đó