Question

Tìm x , biết :

1 + 3 + 5 + ... + x = 36

Answer 1

1 + 3 + 5 + ... + x = 36

ta thấy : ( x - 1 ) : 2 + 1 = x : 2

=> \(\frac{\left(x+1\right).x:2}{2}\)= 36

=> \(\left(x+1\right).x:2=72\)

=> \(\left(x+1\right).x=144\)

Mà 12 . 12 = 144

=> x không thỏa mãn

Answer 2

\(1+3+5+...+x=36\)

\(\left(x+1\right)+\left(3+x-2\right)+...+\left(5+7\right)=36\)

\(\Rightarrow x+1=12\)

\(x=11\)