# Thread: Expressing

1. ## Expressing

Given that $cotA=K$ and that $A$ is acute, express $sec2A$ in terms of $K$

2. Hello Punch
Originally Posted by Punch
Given that $cotA=K$ and that $A$ is acute, express $sec2A$ in terms of $K$
Here are the formulae you'll need:
If $A$ is acute, $\cot A = \frac{\text{adjacent}}{\text{opposite}}=\frac{K}{1 }$, so you can find $\cos A$ using Pythagoras' Theorem.

$\sec 2A = \frac{1}{\cos 2A}$

$\cos2A = 2 \cos^2A -1$
Can you complete it now?

Grandad

3. I tried playing around with formulas and actually solved it using formulas, thanks for your help!