Posted by **Donovan** on Tuesday, January 6, 2009 at 6:46am.

Please help...I'm not understanding trig identities and how to manipulate and express these two problems in their associated functions.

Thanks

a)Express as a function of cos (theta)

2 sin^2(theta) - 1

b)Express as a function of sin (theta) or cos (theta)

tan^2(theta) - 2 sec(theta)sin(theta)

- Trig Identities -
**Reiny**, Tuesday, January 6, 2009 at 9:01am
I will use x instead of theta for easier typing

2 sin^2 x - 1 = cos 2x

do you have to prove this?

if so, then

RS = -cos(2x)

= -cos(x+x)

= -cosxcosx + sinxsinx

= -cos^x + sin^x

= -(1-sin^x_ + sin^x

= 2sin^x - 1

= LS

- Trig Identities -
**Reiny**, Tuesday, January 6, 2009 at 9:05am
for the second

tan^x - 2secxsinx

= sin^2 x/cos^2 x - 2sinx/cosx

= sin^2 x/cos^2 x 2 sinxcosx/cos^2 x

= (sin^2 x - sin 2x)/(1 - sin^2 x)

this is in terms of sinx

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