陈楚醒
幼苗
共回答了15个问题采纳率:86.7% 举报
(3/sin²140-1/cos²140)/2sin10
= (3cos²140-sin²140)/2(sin²140cos²140sin10)
= 4(1+2cos280)/2(sin²280sin10)
= 8(cos60+cos80)/2(sin80sin80cos80)
= 32(cos70cos10)/2(sin80sin160)这里的和差化积很重要额~
= 32(sin20sin80)/2(sin80sin20)
= 32/2
=16
1年前
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