Evaluate
-\frac{\sqrt{3}}{2}\approx -0.866025404
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\cos(180+30)=\cos(180)\cos(30)-\sin(30)\sin(180)
Use \cos(x+y)=\cos(x)\cos(y)-\sin(y)\sin(x) where x=180 and y=30 to obtain the result.
-\cos(30)-\sin(30)\sin(180)
Get the value of \cos(180) from trigonometric values table.
-\frac{\sqrt{3}}{2}-\sin(30)\sin(180)
Get the value of \cos(30) from trigonometric values table.
-\frac{\sqrt{3}}{2}-\frac{1}{2}\sin(180)
Get the value of \sin(30) from trigonometric values table.
-\frac{\sqrt{3}}{2}-\frac{1}{2}\times 0
Get the value of \sin(180) from trigonometric values table.
-\frac{\sqrt{3}}{2}
Do the calculations.
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