Question

Tìm x biết:  1+2+3+...+x=210

Answer 1

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

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

=> x.(x+1)=210.2

=> x.(x+1)=420

=> x.(x+1)=20.21

=> x.(x+1)=20.(20+1)

=> x=20

Answer 2

1+2+3+...+x=420

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

=>x²+x-420=0

=>x²+21x-20x-420=0

=>x(x+21)-20(x+21)=0

=>(x-20)(x+21)=0

vì x>0=>x-20=0

=>x=20

vậy x=20