Question

(1)2/1+6/1+12/1+20/1+30/1+42/1+56/1+72/1+90/1

Answer 1

1,原式=1/(1X2) +1/(2X3) +1/(3X4)+...+1/(9X10)

=(2-1)/(1X2) +(3-2)/(2X3)+(4-3)/(3X4)+...+(10-9)/(9X10)

=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+.-1/10

=1-1/10

=9/10

2,略

3,=1/(4) +1/(4x6) +1/(7x10) +1/(10x13) +1/(13x16)

=1/4+1/2 (6-4)/(4x6) +1/3 (10-7)/(10x7) +1/3(13-10)/(10x13) +1/3 (16-3)/1(16x13)

=1/4+1/2 (6-4)/(4x6) +1/3(1/7-1/16)

=自己算