If $ a^2 + b^2 > 5c^2 $ in a triangle ABC then show c is the smallest side.I tried to solve this by cosine rule but i was not able to find the answer
First, recall that in any triangle, the triangle inequality states that $a+b>c$, $b+c>a$, and $c+a>b$.
Combining this with your condition $a^2+b^2>5c^2$, we have
Now, suppose it were the case that $b\leq c$. Then $2c^2<b^2+bc\leq 2c^2$, a contradiction.
A similar argument holds when $a\leq c$, hence $c$ is indeed the smallest side.