# Trigonometry sin and cos question

#### _Andreas

My teacher told me that

C sin(k x) + D cos(k x) = F sin(k x+gamma)

if neither C nor D is 0 (gamma is a phase angle). How do you derive the expression on the right of the equal sign from that on the left?

Last edited:

#### neutrino

While normally one would say C and D assume certain values, I'll let you find what those values are. Expand the expression on the right and equate the terms on both sides.

#### _Andreas

While normally one would say C and D assume certain values, I'll let you find what those values are. Expand the expression on the right and equate the terms on both sides.
Actually, I just found out that my course book provides the answer to my question. Sorry for bothering! But thanks anyway.

So, note to self: always look in your books before posting questions here.

#### genneth

The relevant trig identity is $$\sin (A+B) = \sin A \cos B + \sin B \cos A$$.

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving