Question

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

Answer 1

x + 2x + 3x + ... + 10x = -165

=> x.(1 + 2 + 3 + ... + 10) = -165

=> x.(10 + 1).10:2 = -165

=> x.55 = -165

=> x = -165:55

=> x = -3