\(x+2x+3x+...+15x=\left(x+15x\right)+\left(2x+14x\right)+\left(3x+13x\right)+\left(4x+12x\right)+\left(5x+11x\right)+\left(6x+10x\right)+\left(7x+9x\right)+8x=7.16x+8x=1200\Leftrightarrow x=\dfrac{1200}{7.16+8}=10\)
Giải:
\(x+2x+3x+...+15x=1200\)
\(x.\left(1+2+3+...+15\right)=1200\)
Số số hạng \(\left(1+2+3+...+15\right)\) : \(\left(15-1\right):1+1=15\)
Tổng dãy \(\left(1+2+3+...+15\right)\) : \(\left(1+15\right).15:2=120\)
\(\Rightarrow x.120=1200\)
\(\Rightarrow x=1200:120\)
\(\Rightarrow x=10\)
Chúc bạn học tốt!