Trignometric Identities

[GrandMaster87]

1. The problem statement, all variables and given/known data
Prove that
\frac{sin3x}{sinx}-\frac{cos3x}{cosx} = 2


2. Relevant equations



3. The attempt at a solution
LHS:\frac{sin(2x+x)}{sin}-\frac{cos(2x+x)}{cosx}

=\frac{sin2x.cosx + cos2x.sinx}{sin}-\frac{cos2xcosx - sin2x.sinx}{cosx}

[GrandMaster87]

thats as far i got...im really new with trig ..caught a wake up call at school so i started working with it...

[njama]

By multiplying \frac{sin3x}{sinx} with \frac{cos(x)}{cos(x)} and \frac{cos(3x)}{cos(x)} with \frac{sin(x)}{sin(x)} you got:

\frac{cos(x)sin(3x)-sin(x)cos(3x)}{sin(x)cos(x)}

What can you spot now? :smile:

[mathie.girl]

1. The problem statement, all variables and given/known data
Prove that
\frac{sin3x}{sinx}-\frac{cos3x}{cosx} = 2


2. Relevant equations



3. The attempt at a solution
LHS:\frac{sin(2x+x)}{sin}-\frac{cos(2x+x)}{cosx}

=\frac{sin2x.cosx + cos2x.sinx}{sin}-\frac{cos2xcosx - sin2x.sinx}{cosx}

Did you try getting a common denominator at this point?

[GrandMaster87]

By multiplying \frac{sin3x}{sinx} with \frac{cos(x)}{cos(x)} and \frac{cos(3x)}{cos(x)} with \frac{sin(x)}{sin(x)} you got:

\frac{cos(x)sin(3x)-sin(x)cos(3x)}{sin(x)cos(x)}

What can you spot now? :smile:

can we expand sin(3x) and cos(3x) using double angle formula?

[kbaumen]

1. The problem statement, all variables and given/known data
Prove that
\frac{sin3x}{sinx}-\frac{cos3x}{cosx} = 2


2. Relevant equations



3. The attempt at a solution
LHS:\frac{sin(2x+x)}{sin}-\frac{cos(2x+x)}{cosx}

=\frac{sin2x.cosx + cos2x.sinx}{sin}-\frac{cos2xcosx - sin2x.sinx}{cosx}

It can also be done onwards from here. Do you know how to expand cos(2x) and sin(2x)?

[njama]

can we expand sin(3x) and cos(3x) using double angle formula?

No.

You can write the nominator:

cos(x)sin(3x)-sin(x)cos(3x)

as

sin(3x-x)=sin(2x)

using the sum difference formula.

Also you can write the denominator

cos(x)sin(x)

as

\frac{sin(2x)}{2}

:smile: