In △ABC, there are:

A+C = 180 B，B+C = 180 A

∵ A, B and C do geometric series.

∴b？ =ac, then c=b? /a

From the sine theorem, we get:

Let a/sinA=b/sinB=c/sinC=k,

sinA=a/k，sinB=b/k

you∶c < a+b

∴b？ /a