In any triangle, the greater side subtends the greater angle.

ABC is a triangle having side AC greater than AB.

To prove.  ∠ABC is also greater than ∠BCA & ∠BAC

Proof

As AC is greater than AB, let AD be made equal to AB. Join BD 

    ∠ADB > ∠DCB                   (ext. ang. > internal & opp. angle). 

    AD = AB                               (construct)

∴ ∇ ABD is isoscelese

∴ ∠ADB = ∠ABD                    (isosceles triangle) 

∠ABD is also greater than angle ACB. Therefore ∠ABC is much greater than ∠ACB

Similarly it can be shown that ∠ABC is much greater than ∠BAC

Thus, in any triangle, the greater side subtends the greater angle.