Question

 Tìm x biết x + 2x + 3x + ⋯ + 10x = -165

Answer 1

 x + 2x + 3x + ⋯ + 10x = -165
\(\Leftrightarrow\)\(\left(1+2+3+4+5+6+7+8+9+10\right)x=-165\)
\(\Leftrightarrow55x=-165\)
\(\Leftrightarrow x=\frac{-165}{55}=-3\)
Vậy, \(x=-3\)

Answer 2

=> { [ 10 . ( 10 + 1 ) ] : 2 } . x = - 165

=> [ ( 10.11 ) : 2 ] . x = - 165

=> ( 110 : 2 ) . x = - 165

=> 55 . x = - 165

=> x = - 165 : 55

=> x = - 3

Vậy x = - 3