Everything is fine with your idea, arbolis. Now, in order to prove that is less than or equal to 5 over C, all you have to do is apply the triangle inequality. You get Since is equal to and over C, you conclude that Done.
If is the frontier of the triangle with vertices , and , show that without calculating the integral.
My attempt : I drew the sketch of .
Then I wrote .
I also wrote that , hence I'm done if I show that for . (Correct me please if I'm wrong saying that , where denotes the frontier of the triangle.)
Here's where I'm stuck. I thought about calculating the image of the interior of the triangle under the transformation but I believe this is a long work. Also it wouldn't finish there. I believe I'm missing something obvious.