10/(1*2)+20/(1*2*3)+30/(1*2*3*4)+40/(1*2*3*4*5)+...+90/(1*2*3*4*5*6*4*8*9*10)=?
问题描述：
10/(1*2)+20/(1*2*3)+30/(1*2*3*4)+40/(1*2*3*4*5)+...+90/(1*2*3*4*5*6*4*8*9*10)=?
答
原式+10/(1*2*3*4*5*6*4*8*9*10)
=10/(1*2)+……+80/1*2*3*4*5*6*4*8*9)+90/(1*2*3*4*5*6*4*8*9*10)+10/(1*2*3*4 *5*6*4*8*9*10)
=10/(1*2)+……+80/1*2*3*4*5*6*4*8*9)+ 100/(1*2*3*4*5*6*4*8*9*10)
=10/(1*2)+……+80/1*2*3*4*5*6*4*8*9)+ 10/(1*2*3*4*5*6*4*8*9)
=…………
=10/(1*2)+10/(1*2)
=10
所以原式=10-10/(1*2*3*4*5*6*4*8*9*10)
=多少自己算