Question

已知f(a)=[sin(-a+π/2)*cos (3π/2-a)*tan(a+5π)]/[tan(-a-π)*sin(a-3π)]

1化简f(a)
2 若a是第三项象限角，且cos(a-3π/2)=1/5，求f(a)的值
3若a=-31π/3，求f(a)的值
过程啊 谢谢了

Answer 1

1. f(a)=[cosa*(-sina)*tana]/[-tana*(-sina)]
=-cosa
2. cos(a-3π/2)=-sina=-1/5
cos^2a=24/25 a是第三项象限角
cosa=-2√6/5
f(a)=2√6/5
3. f(a)=-cos(-31π/3)=-cos(-10π-π/3)=-cosπ/3=-1/2

Answer 2

f(a)=[sin(-a+π/2)*cos (3π/2-a)*tan(a+5π)]/[tan(-a-π)*sin(a-3π)]

=(cosa*(-sinα)*tanα)/((－tanα)*(－sinα))=-cosa
cos(a-3π/2)=－sinα=1/5 f(a)=根号24/5

f(a)=-cos(-31π/3)=-cos(-π/3)=-1/2

Answer 3

1.f(a)=【cosa*（-sina）*tana】/【-tana*(-sina)】=-cosa
2.cos(a-3π/2)=sina=1/5
所以cosa=-1/5，
f（a）=1/5
3.f(a）=cos(-31π/3)=cos(-π/3)=1/2
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