Very difficult Double angle questions needed. (1 Viewer)

[tommykins]

回复: Re: Very difficult Double angle questions needed.

I did the question, and i got the answer pq=-1.
My it is a typo.
Thanks for your help

Haha yeah, after doing parametrics a shit load you actually rmember the properties.

No problems.

 

[tau281290]

arent u the guy that people showed how to do expand those using De Moivre's theorem? (4 unit technique).

Say u want to exapnd cos 6x

(cosx + isinx)^6 = cos6x + isin6x (using De Moivre's theorem)

now expand the LHS using the Binomial Theorem, and the real part willl be the expansion of cos6x.

And are these the sort of questions you're looking for?

http://www.counton.org/alevel/pure/purexatriide.htm

OMG, i spent 40 mins just doing these 4 questions. How do you do them fast in the exam?????

 

[tommykins]

OMG, i spent 40 mins just doing these 4 questions. How do you do them fast in the exam?????

Look LHS and RHS and think of ways to manipulate it to equate each other.

sin2xcosx/2 = sinx - sin³x

We can see that the RHS only has sinx's, so manipulate the left to have sinx's too.

sin2x = 2sinxcosx

LHS = 2sinxcosx.cosx/2 = sinx.cos²x = sinx(1-sin²x) = sinx - sin³x = RHS.

 

[lolokay]

OMG, i spent 40 mins just doing these 4 questions. How do you do them fast in the exam?????

for 1., 2. and 4., simplify the RHS by expanding and factorising and it becomes pretty easy to see how to get from LHS to RHS. I find this helps in many prove/show questions

for the 3rd, basic expansion using 3x = (2x + x) shouldn't take too long

 

[shaon0]

OMG, i spent 40 mins just doing these 4 questions. How do you do them fast in the exam?????

Thos do take quite a long time...took me 10 minutes all of them and in a test which goes for 40minutes or so. You won't be able to finish them all.