ta có:\(x^2y+2x^2y+3x^2y+...+nx^2y=210x^2y\)
\(x^2y\left(1+2+3+4+...+n\right)=210x^2y\)
\(1+2+3+...+n=210x^2y:\left(x^2y\right)\)
\(1+2+3+...+n=210\)
\(\frac{\left(n-1\right):1+1}{2}.\left(n+1\right)=210\)
\(n\left(n+1\right):2=210\)
\(n.\left(n+1\right)=420=20.21\)
vậy n=20
1+2+3+...+n = 210
n(n+1):2=210
n(n+1)=420 =20.21
n =20