If http://thestudentroom.co.uk/latexren...df76930788.png, http://thestudentroom.co.uk/latexren...cedd942305.png show that the extremal is http://thestudentroom.co.uk/latexren...297cb77b55.png

I've done:

http://thestudentroom.co.uk/latexren...ff44eb53f4.png

http://thestudentroom.co.uk/latexren...0bc9b92352.png

Then using Euler's equation we get:

http://thestudentroom.co.uk/latexren...cfcfe46cb0.png

So http://thestudentroom.co.uk/latexren...d43f438260.png

and then I'm stuck from here, I can't see how to turn this is into trigonometric functions? I tried letting http://thestudentroom.co.uk/latexren...0f94906bb0.png which didn't work...

Can anyone please help me?

Thanks in advance!