可以用约减的手法化简瞎穗1/11*1/13+1/13*1/15++1/15*1/17+1/17*1/19
=(1/2)(2/11*1/13+2/13*1/15+2/15*1/17+2/17*1/19)
=(1/2)(1/11-1/13+1/13-1/15+1/15-1/袭腊17+1/17-1/19)
=(1/2)(1/磨禅卜11-1/19)
=4/209
1/11×1/13＋1/13×1/15＋1/15×1/17＋1/17×1/19
=1/2×（1/11－1/13）＋1/2×（1/13－1/15）＋1/2×（1/15－巧配1/17）＋1/2×（1/17－1/19）
=1/手宽孙2×（1/11－1/13＋1/13－1/15＋1/15－1/17＋1/17－毕链1/19）
=1/2×（1/11－1/19）
=1/2×8/209
=4/209
1/11*1/13+1/13*1/弯如瞎15+1/15*1/17+1/17*1/19=1/2(1/橡橡11-1/13+1/13-1/埋空15+1/15-1/17+1/17-1/19)=1/2(1/11-1/19)=4/209
=1/2(1/11-1/13)+1/2(1/旁梁13-1/15)1/2(1/运闹运15-1/17)+1/2(1/17-1/19)
=1/弯基2(1/11-1/19)
=4/209
1/11*1/13+1/13*1/15++1/15*1/17+1/17*1/19
=(1/悔誉2)(2/11*1/13+2/13*1/15+2/15*1/州前大17+2/17*1/19)
=(1/2)(1/11-1/13+1/13-1/15+1/15-1/17+1/17-1/19)
=(1/2)(1/11-1/19)
=4/册竖209