SOLUTION: Which expression is equivalent to: cos( theta + pi/2) a. cos theta b. - cos theta c. sin theta d. - sin theta

Algebra ->  Trigonometry-basics -> SOLUTION: Which expression is equivalent to: cos( theta + pi/2) a. cos theta b. - cos theta c. sin theta d. - sin theta      Log On


   

Question 348820: Which expression is equivalent to: cos( theta + pi/2)
a. cos theta
b. - cos theta
c. sin theta
d. - sin theta
Found 3 solutions by stanbon, ewatrrr, CarterC:
Answer by stanbon(75887) About Me  (Show Source):
You can put this solution on YOUR website!
Which expression is equivalent to: cos( theta + pi/2)
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Thinking about it.
Let theta = (pi)/6
Then cos((pi/6)+(pi/2)) = cos(pi/6)cos(pi/2))+sin(pi/6)sin(pi/2)
= 0+sin(p/6)

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Ans: sin(theta)
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Cheers,
Stan H.
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a. cos theta
b. - cos theta
c. sin theta
d. - sin theta

Answer by ewatrrr(24785) About Me  (Show Source):
You can put this solution on YOUR website!


Hi,



Answer C would be the correct choice

Answer by CarterC(2) About Me  (Show Source):
You can put this solution on YOUR website!
I just thought I'd register and write a solution because both of these solutions are incorrect (and this is the top result on google!)
Both solutions used the incorrect double angle formula- it should be:
cos(A + B) = cosAcosB - sinAsinB
Hence the answer is -sin(theta)
More conceptually, consider what happens to the cos(theta) curve if you're adding pi/2 to values of theta.
Theta=0: cos(0 + pi/2) = 0 (note both sin(0)=0 and -sin(0)=0)
Theta=pi/2: cos(pi/2 + pi/2) = -1 (note -sin(pi/2)=-1)
Hence -sin(theta) is the curve equivalent to cos(theta+pi/2)