Question

tìm x biết  1+2+3+4+5+...+x=45

Answer 1

1+2+3+4+5+...+x=45

1+2+3+4+5+6+7+8+x=45

x=45-8-7-6-5-4-3-2-1

x=9

Answer 2

1+2+3+4+5+...+x=45

(x+1)*x:2=45

(x+1)*x:2=(9+1)*9:2

=> x=9

 

Answer 3

1+2+3+4+5+...+x=45

(x+1)*x:2=45

(x+1)*x:2=(9+1)*9:2

=> x=9

Answer 4

1+2+3+...+x=45

1+2+3+4+5+6+7+8+x=45

x=45-1-2-3-4-5-6-7-8

x=9

Vậy x=9