Question

( x-1 ) + ( x-2 ) + ( x-3 ) + .... + ( x-20 ) = 90

Answer 1

(x-1)+(x-2)+(x-3)+...+(x-20)=90

= (x+x+x+...+x)-(1+2+3+...+20) = 90

= 20x-210 = 90

20x = 90+210

20x = 300

x = 300:2

x = 150

Answer 2

(x-1)+(x-2)+(x-3)+...+(x-20)=90

Vì :có (20-1):1+1=20(cặp số) 

=>x-1+x-2+x-3+...+x-20=90

=>(20.x)-(1+2+3+...+20)=90

Đặt A=1+2+3+...+20

    A có 20số hạng

=>(20+1).20:2=210

=>(20.x)-210=90

=>20.x=90+210

=>20. x=300

=> x=300:20

=> x=15

  Vậy x = 15