Thread: trig proof

In tirangle ABC, angle C is . Show that



I think I am suppose to be using the half angle formula;

I did draw a right angle triangle, labelling the hypotenuse c and got;



Using the half angle formula, wouldn't I just sub in what cosA = a\c, but this does not work.

Can some point me to the right direction?

Thanks.

You have made two mistakes:

I think I am suppose to be using the half angle formula;

and

.

In fact we have

and

oh right, thanks.


Can you also tell me why my labelling of the right angled triangle is wrong?

I labelled the other two angles A and B and the sides opposite them a , b .

Your labelling is right.

But