这样吧,裂项求和。
1/2x3=1/2-1/3 , 1/3x4=1/3-1/4 所以依次类推。
原式=1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6 = 1/2 - 1/6 = 1/3.
原式=(3-2)/(2*3)+(4-3)/(3*4)+(5-4)/(4*5)+(6-5)/(5*6)
=1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6
=1/2-1/6=1/3
1/(2*3)+1/(3*4)+1/(4*5)+1/(5*6)=(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+(1/5-1/6)=1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6=1/2-1/6=1/3
=1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6
=1/2-1/6
=1/3
1/(2*3)+1/(3*4)+1/(4*5)+1/(5*6)=
=1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6
=1/2-1/6
=1/3