Question

tìm x thuộc N, biết 1+ 2+...+x=55

Answer 1

1 + 2 + ... + x = 55

( 1 + x ) . x : 2 = 55

( 1 + x ) . x = 110

( 1 + x ) . x = 11.10

Vậy x = 10

Answer 2

1+2+3+4+5+6+7+8+9+10=55

1+2+3+4+5+6+7+8+9+X=55

-> x=10