Question

Số nguyên x thỏa mãn (-1)+(-2)+(-3)+...+x= -120 lá

Answer 1

=>-(1+2+3+...+x)=-120

=>1+2+3+...+x=120

=>(x+1)x/2=120

=>x(x+1)=240

=>x(x+1)=15.16

=>x=15

Answer 2

=>-[1+2+3+...+(-x)]=-120

=>1+2+3+...+(-x)=120

=>-(x+1).(-x)/2=120

=>(-x).[-(x+1)]=240

=>(-x)[-(x+1)]=-15.(-16)

=>x=-15

Answer 3

1+2+3+.....+X=120

[( X+1 ) x X] : 2 = 120

( X +1 ) x X = 120 x 2

( X + 1 ) x X = 240

( X + 1 ) x X = 15 x 16

X = 15