(1+1/11+1/13+1/17)*(1/11+1/13+1/17+1/19)-(1+1/11+1/13+1/17+1/19)(1/11+1/13+1/17)
快速
(1+1/11+1/13+1/17)*(1/11+1/13+1/17+1/19)-(1+1/11+1/13+1/17+1/19)(1/11+1/13+1/17)
快速
1/11+1/13+1/17=a
(1+a)*(a+1/19)-(1+a+1/19)a
=a+1/19+aa+a*1/19-a-aa-a*1/19
=1/19
令1/11+1/13+1/17=想则上式可变形为
(x+1)(x+1/19)-(x+1+1/19)x
去括号化简=1/19
结果就是1/19
设1/11+1/13+1/17=M,1/11+1/13+1/17+1/19=N.
(1+1/11+1/13+1/17)*(1/11+1/13+1/17+1/19)-(1+1/11+1/13+1/17+1/19)(1/11+1/13+1/17)
=(1+M)N-(1+N)M=N+MN-M-MN=N-M=(1/11+1/13+1/17+1/19)-(1/11+1/13+1/17)=1/19