Cos
 Usage
• Cos[z]返回 z的余弦值。
Notes
• Cos的参数假定为弧度。(用Degree相乘从度数转换.) • 当它的参数是  的一个简单有理数倍数， Cos 自动计算；对更复杂的有理倍数，有时使用 FunctionExpand。
 Further Examples
You can compute exact values of trigonometric functions.
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This is the derivative.
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This is the indefinite integral.
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This is the series expansion around  to order  .
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You can use FunctionExpand to evaluate Cos at complicated rational multiples of  .
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