Greatest value of (cot(A/2)+cot(B/2)+cot(C/2))/(cotA+cotB+cotC) in triangleABC

292 Views Asked by At

enter image description here

I am struck at the last step. Please help me to proceed further.

1

There are 1 best solutions below

0
On BEST ANSWER

Cauchy-Schwarz, or just $(a-b)^2\ge0$, implies $$ a^2+b^2\ge 2ab,$$ and so on for $b,c$ and $c,a$. Add them all up we got $$2(a^2+b^2+c^2) \ge 2(ab+bc+ca),$$ which means $$ \frac{ab+bc+ca}{a^2+b^2+c^2}\le 1.$$ That would solve your last step.

And Xander was right, try to post in LaTeX markup.