cos(–180°–a)可化为 sin(–a–180)等于多少
cos(a\u201390\u00b0)\u4e3a\u4ec0\u4e48\u7b49\u4e8ecos(90\u00b0\u2013a)cos(a\u201390\u00b0)\u4e3a\u4ec0\u4e48\u7b49\u4e8ecos(90\u00b0\u2013a)?
\u4e09\u89d2\u51fd\u6570\u7684\u89d2\u5ea6\u8fd0\u8f6c\u5728\u56db\u4e2a\u8c61\u9650\u4e4b\u4e2d\uff0c\u51b3\u5b9a\u4e86\u7b80\u5316\u540e\u6b63\u8d1f\u5173\u7cfb\uff0c\u6bd4\u5982
\u4e0a\u8ff0
cos(a\u201390\u00b0)\u4ece\u8868\u793a\u5f0f\u6765\u770b\u5f53a\u5c0f\u4e8e90\u5ea6\uff0c\u5e94\u5f53\u5728\u7b2c\u56db\u8c61\u9650\uff0c\u4f60\u53ef\u4ee5\u753b\u4e00\u4e0b\u5b83\u7684\u89d2\u5ea6\u662f90-a,\u5b83\u7684\u503c\u662fcos(90-a)
\u5982\u56fe
cos（-180°-α）
=cos[-(180°+α)]
=cos（180°+α）
=-cosα
-cosa
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