简算1/2*3+1/3*4+1/4*5+.+1/18*19+1/19*20
问题描述：
简算1/2*3+1/3*4+1/4*5+.+1/18*19+1/19*20
答
1/（n（n+1））=1/n-1/(n+1)
以此类推
1/2*3+1/3*4+1/4*5+......+1/18*19+1/19*20
=1/2-1/3+1/3-1/4+...+1/18-1/19+1/19-1/20
=1/2-1/20
=9/20
答
1/2*3+1/3*4+1/4*5+.+1/18*19+1/19*20
=1/2-1/3+1/3-1/4+...+1/18-1/19+1/19-1/20
=1/2-1/20
=9/20