题目内容
求值:cos20°+cos100°+cos140°.
解:原式=cos20°+cos(120°-20°)+cos(120°+20°)
=cos20°+2cos120°·cos20°
=cos20°-cos20°
=0.
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求值
=（ ）
| cos20° | ||
cos35°
|
| A、1 | ||
| B、2 | ||
C、
| ||
D、
|
题目内容
求值:cos20°+cos100°+cos140°.
解:原式=cos20°+cos(120°-20°)+cos(120°+20°)
=cos20°+2cos120°·cos20°
=cos20°-cos20°
=0.
| cos20° | ||
cos35°
|
| A、1 | ||
| B、2 | ||
C、
| ||
D、
|