Core 3 Maths - Double Angle Question - Am I Right?

Squarehed · 13 Jan 2008 at 22:47

Hi guys, just wanting to check I'm right in my method here as I don't have the mark scheme for this practice paper to hand.

Question 8(a) of June 2006 C3 paper:

Given that cos A= 3/4, where 270°< A <360°, find the exact value of sin 2A

I know double angle formula: sin2A = 2sinAcosA

My working was like this:

If cosA = 3/4. Pythagoras gives us a triangle of √7, 3, 4

Therefore sinA = √7/4

Putting these into sin2A = 2sinAcosA gives: 2(√7/4)(3/4)

This gives us the value of sin2A as: (3√7/8)

Have I gone about this the right way or is it wrong to assume that I can use pythagoras?

Squarehed · 13 Jan 2008 at 23:17

Oh yeah, past papers are where its at. I'm trying to keep my average up from last year w/ core 1,2 and s1 coming in at 98/100 UMS pts average. Don't feel as confident for this one, but its Thursday afternoon so will have hopefully got sorted by then.

As for this question, I had used my calculator at first but remembered halfway through that it'd probably round the irrational values into decimals. I am right in thinking I can always assume we have a right angled triangle for this type of question?

Thanks for the help

Squarehed · 14 Jan 2008 at 00:11

Ugh, I'm getting things confused with something we did way back at GCSE. With trig in non-rt angled triangles? Now I'm all embarrassed! I've been out of c3 practice after teaching myself D1 over the hols and revising other subjects :s

Thanks again for your help guys. Night!

Squarehed · 14 Jan 2008 at 16:41

See what I got wrong there. Is there anyone else who is moved to near-violence by the sound of that man's voice, it drives me insane!