\(1+2+3+...+n=\frac{\left(1+2+...+n\right)+\left(n+\left(n-1\right)+...+1\right)}{2}.\)

\(=\frac{\left(n+1\right)+\left(n+1\right)+...+\left(n+1\right)}{2}.\left(có.n.nhóm.n+1\right)\)

\(=\frac{n\left(n+1\right)}{2}.\)

tìm n nhak các bạn

x^2y*(1+2+3+...+n)=210x^2y

1+2+3+...+n=210

[(n-1):1+1]:2*(n+1)=210

n*(n+1):2=210

n*(n+1)=420=20*21

suy ra n=20

k mik nhe

\(x^2y+2x^2y+3x^2y+....+nx^2y=210x^2y\)

\(x^2y\left(1+2+3+...+n\right)=210x^2y\)

\(1+2+3+...+n=210\)

\(\frac{n\left(n+1\right)}{2}=210\)

\(n\left(n+1\right)=420\)

\(n\left(n+1\right)=20.21\)

\(\Rightarrow n=20\)

x^2.y+2x^2.y+3x^2.y+...+n.x^2y=210x^2.y

x^2.y(1+2+3+..+n)=210x^2.y

1+2+3+..+n=210

=>(n+1)(n-1+1)/2=210

(n+1)n/2=210

(n+1)n=420=21.20

=>n+1=21

n=20

Nguyễn Huy Tú giúp mình với nha

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

n=20 nha rồi mình kik hộ cho

bạn coi lại đề đi nhá, không tìm được n