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Find a formula for \(\cos \left(\theta+\frac{\pi}{2}\right)\).

Short Answer

Expert verified
The formula for \(\cos\left(\theta+\frac{\pi}{2}\right)\) is: \(\cos\left(\theta+\frac{\pi}{2}\right) = -\sin(\theta)\).

Step by step solution

01

Write down the angle addition formula for cosine

Our starting point is the angle addition formula for cosine: \[\cos(\alpha + \beta) = \cos(\alpha)\cos(\beta) - \sin(\alpha)\sin(\beta)\]
02

Replace \(\alpha\) with \(\theta\) and \(\beta\) with \(\frac{\pi}{2}\)

Now we replace \(\alpha\) with \(\theta\) and \(\beta\) with \(\frac{\pi}{2}\) in the formula: \[\cos(\theta + \frac{\pi}{2}) = \cos(\theta)\cos\left(\frac{\pi}{2}\right) - \sin(\theta)\sin\left(\frac{\pi}{2}\right)\]
03

Use trigonometric values of the angles

Recall that \(\cos\left(\frac{\pi}{2}\right) = 0\) and \(\sin\left(\frac{\pi}{2}\right) = 1\). Now we substitute these values into the formula: \[\cos(\theta + \frac{\pi}{2}) = \cos(\theta)(0) - \sin(\theta)(1)\]
04

Simplify

Now we simplify the formula: \[\cos(\theta + \frac{\pi}{2}) = - \sin(\theta)\] So, the formula for \(\cos \left(\theta+\frac{\pi}{2}\right)\) is: \[\cos \left(\theta+\frac{\pi}{2}\right) = - \sin(\theta)\]

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