1/2×3 = 1/2 - 1/3 ;1/3×4 = 1/3 - 1/4 ; 1/10×11 = 1/10-1/11;
请按规律计算:1/2+1/6+1/12+1/20+1/42+1/56+1/72+1/90的值吗?