求值：3−sin70°2−cos210°．
问题描述：
求值：
． 3−sin70° 2−cos210°
答
原式=
=3−sin(90°−20°) 2−
1+cos20° 2
=3−cos20° 2−
1+cos20° 2
=6−2cos20° 4−1−cos20°
=2．2(3−cos20°) 3−cos20°