1/1*3+1/2*4+1/3*5+1/4*6+1/5*7+1/6*8------1/17*19+1/18*20=1/2（1-1/3+1/2-1/4+1/3-1/5+1/4-1/6+%1/17-1/19+1/18-1/20）=1/2×（1-1/19+1/2-1/20）=1/2×（18/19+9/20）=1/2×531/380=531/760