a,x1,x2...,xn,b成等比数列
则有ab=x1xn=x2x(n-1)=...
设S=x1*x2*...*xn
则S=xn*x(n-1)*...*x1
两式相乘得:S²=(x1xn)^n
S²=(ab)^n
S=√(ab)^n