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Core 4 trig identities question

I'm really stuck on this question, can someone help please,

By using the expansion of cos(A+B) prove the identity
cos(3x)=4cos^3(x) -3cos(x)

Thanks in advance! :smile:

Reply 1

Notnek

Original post by mica-lwe

I'm really stuck on this question, can someone help please,

By using the expansion of cos(A+B) prove the identity
cos(3x)=4cos^3(x) -3cos(x)

Thanks in advance! :smile:

What have you tried? Post your working.

Reply 2

mica-lwe

Original post by notnek

What have you tried? Post your working.

Might have thrown that piece of paper out couldn't read it, was too full :colondollar:

I've tried changing cos(3x) to cos^3(x)-sin^3(x) and 3cos^3(x)-1

Reply 3

Mr M

Study Forum Helper

Original post by mica-lwe

Might have thrown that piece of paper out couldn't read it, was too full :colondollar:

I've tried changing cos(3x) to cos^3(x)-sin^3(x) and 3cos^3(x)-1

That's not good. You are making up rules that don't exist.

Start with cos(3x)=cos(2x + x)

Reply 4

mica-lwe

Original post by Mr M

That's not good. You are making up rules that don't exist.

Start with cos(3x)=cos(2x + x)

I was using my notes for double angle identities, I will try cos(2x+x) but why would the other identities not work? I thought you would be able to use them but I was just doing it wrong?

Reply 5

Mr M

Study Forum Helper

Original post by mica-lwe

I was using my notes for double angle identities, I will try cos(2x+x) but why would the other identities not work? I thought you would be able to use them but I was just doing it wrong?

Because you are trying to apply double angle rules to triple angles. They are completely different.

Check for yourself. Try substituting values into your formula.

Reply 6

mica-lwe

Original post by Mr M

Because you are trying to apply double angle rules to triple angles. They are completely different.

Check for yourself. Try substituting values into your formula.

Ohh okay I see now