求[3/(sin140)^2 -1/(cos140)^2]*[1/(2sin10)]的值?(角度制,^表示次方)
题目
求[3/(sin140)^2 -1/(cos140)^2]*[1/(2sin10)]的值?(角度制,^表示次方)
答案
原式=(3cos²40-sin²40)/[2sin²40cos²40sin10]
=2(√3cos-sin40)(√3cos40+sin40)/[sin²80sin10]
=8sin(60-40)sin(60+40)/(cos²10sin10)
=16sin20sin100/(cos10sin20)
=16cos10/cos10
=16
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