Homework Help: Pre Calc.

Posted by Patrick on Wednesday, April 22, 2009 at 10:54am.

Use the sum or difference identity to find the exact value of sin255 degrees.

My answer: (-sqrt(2)- sqrt(6)) / (4)

Find the value of tan (alpha-beta), if cos alpha= -3/5, sin beta= 5/13, 90<alpha<180, and 90<beta<180.

My answer: -33/56

Which expression is equivalent to cos(pi-theta)?

Is it -cos theta?

Which expression is not equivalent to cos 2theta?

a. cos^2 theta- sin^2 theta
b. 2cos^2 theta - 1
c. 1 - 2sin^2 theta
d. 2sin theta cos theta

I would pick D.

Pre Calc. - Reiny, Wednesday, April 22, 2009 at 11:23am
correct on all counts, good job

For questions on finding "exact" values for certain trig ratios, a quick way to check your result is to simply evaluate your answer with a calculator, then compare it to the calculator result for the initial trig ratio

e.g. for yours I found (-sqrt(2)- sqrt(6)) / (4) to be -.9659258
and sin 255 to be -.9659258 , not bad eh?

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