# Factor Formulae

1. Jun 24, 2011

### Googl

Hi all,

I have an exam today but the questions which will come up I have never come across before. Some of the questions involve the Factor Formulae which I have never seen before so I was wondering whether you might give some examples and perhaps links to other pages where I can take a long revision.

1. Using the factor formulae evaluate the integrals of the type (sinx cos3x)dx

2. Use the factor formulae to prove trig identities.

I have a general understanding of Trigonometry and I have just completed my Calculus.

We've just been told today about this exam and I have no idea how the factor formulae looks like and how to use it.

Thanks.

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

For prove trig using the Factor formulae:

Prove that: 4(cosx + cos2x)sin3xsinx = cos4x - cos8x

My take:

=2(cosx + cos2x) 2sin3xsinx
Reverse using factor formulae
=2(cosx + cos2x)(cos4x - cos2x)

2. Jun 24, 2011

### tiny-tim

Hi Googl!
nooo, wrong direction

general rule of thumb: always try to make things simpler, not more complicated!

sinsin is simpler than (cos + cos), so use the factor formula https://www.physicsforums.com/library.php?do=view_item&itemid=18" on everything except the sin3xsinx

Last edited by a moderator: Apr 26, 2017