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Hi! I want to have precision about co-functions in trigonometry. As I know,...

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havivdiba | (Level 3) eNoter

Posted August 16, 2013 at 9:51 PM via web

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Hi! I want to have precision about co-functions in trigonometry.

As I know, cos(90-theta)= sintheta; cos(180-theta)=-costheta;

cos(360-theta)= costheta

My question is, what's the answer for cos or sin(270-theta),

cos or sin(450-theta)?

Thank you very much

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baxthum8 | High School Teacher | (Level 3) Associate Educator

Posted August 16, 2013 at 10:38 PM (Answer #1)

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So far what you know is correct. So I underatand that you are looking for the angles 270 and 450.  Well since 270 is the (-) y-axis you could use your relationships that you understand for 90 which is the (+) y-axis.

Since cos (90-`theta)=sintheta`

then cos (270-`theta)= -sintheta`

Since sin(90-`theta)=costheta`

then sin(270-`theta)= -costheta`

Since 450 degrees is a coterminal angle for 90 degrees (90+360=450)

cos (450-`theta)=sintheta`

sin (450-`theta)=costheta`

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