# HNC math question.

Can someone help me with this question. I dont really understand Trigonometric identities.

How do I develop an alternative expression for Vo = Vc +[(Vc *Va/8] using Trigonometric identities to show that the output from the multiplier circuit comprises frequency components at 230kHz, 250kHz and 270kHz.
When:
Vc =8cos(2𝜋×250000×t)
Va=5cos(2𝜋×20000×t)
Original post by weeliam9
Can someone help me with this question. I dont really understand Trigonometric identities.

How do I develop an alternative expression for Vo = Vc +[(Vc *Va/8] using Trigonometric identities to show that the output from the multiplier circuit comprises frequency components at 230kHz, 250kHz and 270kHz.
When:
Vc =8cos(2𝜋×250000×t)
Va=5cos(2𝜋×20000×t)

I don't know what trig identities you are given for HNC exams, but at A level the relevant identities are:

cos(A + B) = cosAcosB - sinAsinB
cos(A - B) = cosAcosB + sinAsinB

If you add those two identities together you should end up with an expression for cosAcosB, which is what you need for Vc * Va
Original post by weeliam9
Can someone help me with this question. I dont really understand Trigonometric identities.

How do I develop an alternative expression for Vo = Vc +[(Vc *Va/8] using Trigonometric identities to show that the output from the multiplier circuit comprises frequency components at 230kHz, 250kHz and 270kHz.
When:
Vc =8cos(2𝜋×250000×t)
Va=5cos(2𝜋×20000×t)

Did you get anywhere with this?