Question

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

Answer 1

1 + 2 + 3 + ... + x = \(\frac{x\times\left(x+1\right)}{2}=210\)

=> \(x\times\left(x+1\right)=420\)

\(x\times\left(x+1\right)=20\times21\)

Vậy x = 20