Evaluate
-\frac{\sqrt{3}}{2}\approx -0.866025404
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\cos(90+60)=\cos(90)\cos(60)-\sin(60)\sin(90)
Use \cos(x+y)=\cos(x)\cos(y)-\sin(y)\sin(x) where x=90 and y=60 to obtain the result.
0\cos(60)-\sin(60)\sin(90)
Get the value of \cos(90) from trigonometric values table.
0\times \frac{1}{2}-\sin(60)\sin(90)
Get the value of \cos(60) from trigonometric values table.
0\times \frac{1}{2}-\frac{\sqrt{3}}{2}\sin(90)
Get the value of \sin(60) from trigonometric values table.
0\times \frac{1}{2}-\frac{\sqrt{3}}{2}\times 1
Get the value of \sin(90) from trigonometric values table.
-\frac{\sqrt{3}}{2}
Do the calculations.
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