Question

Tìm x biết : 

     x+2x+3x+....+10x=-165

Answer 1

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

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

tổng trong ngoặc=1+2+3+..+10=[10.(10+1)]:2=55

=>x.55=-165

=>x=-3

Vậy x=-3

Answer 2

x+ 2x + 3x+  ..... + 10x = -165

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

X x 55 = -165

X  = -165 : 55 = -3