Question

1+2+3+.........+x=210

Answer 1

1 + 2 + 3 + ... + x = 210

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

=> (1 + x).x = 210.2

=> (1 + x).x = 420

=> (1 + x).x = 21.20

=> x = 20

vậy x = 20

Answer 2

1 + 2 + 3 + ... + x = 210

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

=> (1 + x).x = 210.2

=> (1 + x).x = 420

=> (1 + x).x = 21.20

=> x = 20

vậy x = 20