（1-1/2²）（1-1/3²）（1-/4²）...（1-1/n²）
=(1-1/2)(1+1/2)(1-1/3)(1+1/3)(1-1/4)(1+1/4) *.*(1-1/n)(1+1/n)
= 1/2*3/2*2/3*4/3*3/4*5/4*.*(n-1)/n*(n+1)/n
= 1/2*(n+1)/n
= (n+1)/(2n)
x²+2x+1+y²-8y+16=0
(x+1)^2+(y-4)^2=0
x=-1,y=4
y/x=4/(-1)=-4