Evaluate
\frac{\sqrt{2}}{2}\approx 0.707106781
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\cos(-\frac{1}{4}\pi )
Combine \frac{\pi }{4} and -\frac{\pi }{2} to get -\frac{1}{4}\pi .
\cos(\frac{1}{4}\pi )
Use the property \cos(-x)=\cos(x).
\frac{\sqrt{2}}{2}
Get the value of \cos(\frac{1}{4}\pi ) from trigonometric values table.
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