（x+y)+(2x+1/1*2y)+(3x+1/2*3y)+.+(9x+1/8*9y) x=2 y=90求和
问题描述：
（x+y)+(2x+1/1*2y)+(3x+1/2*3y)+.+(9x+1/8*9y) x=2 y=90求和
答
（x+y)+(2x+1/1*2y)+(3x+1/2*3y)+.....+(9x+1/8*9y)
=(x+2x+3x+...9x)+(y+1/1*2y+1/2*3+...1/8*9y)
=(1+2+..9)x+(1+1-1/2+1/2-1/3+..+1/8-1/9)y
=45x+(2-1/9)y
=45x+17/9y
=45*2+17/9*90
=90+170
=260
答
（1+2+3+...+9）x+(1+(1-1/2)+(1/2-1/3)+...+(1/7-1/8)+(1/8-1/9))y
=50x+(1+1-1/9)y
=100+270
=370