Thread: Somewhat basic Cosine rule problem. Help appreciated

First part of the question was easy,

The second part is where i'm stuck.

What I have tried :
-Make d the subject in both cases and have one equal to each other (All that happens is that everything cancels each other out and I get 0=0)
-Make d^2 the subject (I don't know why I tried this, same result as before >.>)

Completely lost as to how to solve this problem.
Any help would be appreciated!
(Sorry for poor diagram etc. in advance)

Click the image to make it larger.

Because alpha +beta=180 then cos(alpha)=-cos(beta), so you can make an equation using results from the first part.

Originally Posted by biffboy

Because alpha +beta=180 then cos(alpha)=-cos(beta), so you can make an equation using results from the first part.

Could you please explain this part to me?
Thanks for the reply ^_^

When 2 angles add up to 180 they have the same numerical value but one is the negative of the other eg cos120 =-cos60, cos110=-cos70 Check some values on your calculator.

Originally Posted by biffboy

When 2 angles add up to 180 they have the same numerical value but one is the negative of the other eg cos120 =-cos60, cos110=-cos70 Check some values on your calculator.

Thanks, I understand completely now.

Originally Posted by biffboy

When 2 angles add up to 180 they have the same numerical value but one is the negative of the other eg cos120 =-cos60, cos110=-cos70 Check some values on your calculator.

That's only true for the Cosine and Tangent functions and their reciprocals.









However,