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原式=1/(2sin10)-2sin70
=cos10/(2sin10cos10)-2sin70
=cos10/sin20-2sin70
=(cos10-2sin70sin20)/sin20
=(cos10-2cos20sin20)/sin20
=(cos10-sin40)/sin20
=(cos10-cos50)/sin20
=〔cos(30-20)-cos(30+20)〕/sin20
=2sin30sin20/sin20
=1
1年前
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