Question

tìm x thuộc n biết 2+4+6+...+2x=156

Answer 1

\(2+4+6+...+2x=156\)

=>\(1+2+3+...+x=78\)

=>\(\dfrac{x\left(x+1\right)}{2}=78\)

=>\(x\left(x+1\right)=78\cdot2=156\)

=>\(x^2+x-156=0\)

=>(x+13)(x-12)=0

=>\(\left[{}\begin{matrix}x=-13\left(loại\right)\\x=12\left(nhận\right)\end{matrix}\right.\)