Given: c∉ bd ,△abc d∈ ray bc ,ab=ac=bc prove: bd> da> ab

Triangle ABC is equilateral, because AB=BC=AC=a. Then

m∠A=m∠B=m∠C=60°.

Let point D lie on the ray BC to the right from points B and C and let CD=x. Then BD=a+x, AB=a.

Consider triangle ACD. In this triangle, m∠ACD=180°-m∠ACB=180°-60°=120°.

By the cosine theorem,

Since then

and

Therefore, you get double inequality

or