^2=ac,
1/(a+b)+1/(b+c)
=(a+c+2b)/(a+b)(b+c)
=(a+c+2b)/(ab+ac+b²+bc)
=(a+c+2b)/[(a+c)b+2b²]
=(a+c+2b)/[b(a+c+2b)]
=1/b
(1/2b)*2=1/b
即:1/(a+b)+1/(b+c)=2*1/b
所以,1/a+b,1/2b,1/b+c成等差数列
^2=ac,
1/(a+b)+1/(b+c)
=(a+c+2b)/(a+b)(b+c)
=(a+c+2b)/(ab+ac+b²+bc)
=(a+c+2b)/[(a+c)b+2b²]
=(a+c+2b)/[b(a+c+2b)]
=1/b
(1/2b)*2=1/b
即:1/(a+b)+1/(b+c)=2*1/b
所以,1/a+b,1/2b,1/b+c成等差数列