Question

12.    Tìm x biết : 1 + 2 +3 + 4+ … + x = 1035

Answer 1

=>x(x+1)/2=1035

=>x^2+x=2070

=>x^2+x-2070=0

=>x=45

Answer 2

\(1+2+3+4+...+x=1035\)

\(x\times\left(x+1\right):2=1035\)

\(x\times\left(x+1\right)=1035\times2\)

\(x\times\left(x+1\right)=2070\)

\(x\times\left(x+1\right)=45\times46\)

\(x=45\)