trig - matric exam and i am stuck!!!

nadias · New Member Offline Joined: 07 Nov 2009 Posts: 2

I'm studying for my matric exam in math this monday. PLEASE help someone!!!
I am in the middle of an equation and I have the answer but I don't understand how to get from step 2 to step 3!?
Here is the sum in full:

find(without a calculator) the value of:
1 sin^2(-100) / cos190sin40cos400
2 = (-sin80)^2 / (-cos10)sin40cos40
3 = sin^280 / (-sin80)(1/2)(sin80)
4 = -2

My calculator gives the same answer as the memo but I still don't understand how sin40 times cos40 can give (1/2)sin80??

Can anyone please help?

**Posted:** Sat Nov 07, 2009 4:12 am

tonypr

$\sin 80=2\sin 40\cos 40$ by the Double Angle Formula for Sin, therefore, $\dfrac{\sin 80}{2}=\sin 40\cos 40$

**Posted:** Sat Nov 07, 2009 6:39 am

_________________
Math in trivial problem solving

nadias · New Member Offline Joined: 07 Nov 2009 Posts: 2

thanks a lot TonyP, I only realize now how incredibly easy that actually is, can't believe I didn't think of that. Guess I'll have to keep practising...
Dry

**Posted:** Sat Nov 07, 2009 9:38 am