a level maths first principle question

Original post by esha06

i have no idea how to start this other than expanding cos(A±B) to cosAcosB-+sinAsinB (idk how to do the inverted plus minus)

and I'm not sure if that's even how you start it

Do you know how to start any differentiation from first principles question? E.g. do you know the formula?

Reply 2

OP

Original post by Notnek

Do you know how to start any differentiation from first principles question? E.g. do you know the formula?

the (x+h)-x/h?

(edited 3 months ago)

Reply 3

Original post by esha06

the (x+h)+x/h?

Well it looks something like that. Do you know how to use it for a simpler question like finding the derivative of x^2?

Reply 4

OP

Original post by Notnek

Well it looks something like that. Do you know how to use it for a simpler question like finding the derivative of x^2?

yeah you just do like (x+h)^2-x^2/h and then simplify and make h=0 I think

(edited 3 months ago)

Reply 5

Original post by esha06

yeah you just do like (x+h)^2+x^2/h and then simplify and make h=0 I think

The terms on the numerator should be separated by minus instead of plus but other than that it’s correct. So try applying the same formula to cos(x) and post what you get.

(edited 3 months ago)

Reply 6

OP

Original post by Notnek

The terms should be separated by minus instead of plus but other than that it’s correct. So try applying the same formula to cos(x) and post what you get.

Reply 7

Original post by esha06

Correct. Now expand the cos as you mentioned before. Then think about the two results that are given to you in the question.

This is one of the standard proofs that is worth learning off by heart for the final exam.

Reply 8

OP