Jailiealia B.

asked • 02/26/21

Find the value(s) of θ such that cos (θ) = -1/2 in the interval -270° < θ < -90°

Wendy D.

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Just giving you the answer will not help you. Think about and study the unit circle. You should have a copy of it in your geometry notebook and should carry it with you. Memorize the value of the trig functions for certain key angles (0, 30, 45, 60, 90). Figure out what quadrants would have angles with a negative cosine.
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02/26/21

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Raymond B. answered • 02/26/21

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the interval is quadrants II and III


or the same as 90 < theta < 270 or pi/2 < theta < 3pi/2


cosine theta is negative in either quadrant


cos(theta) = -1/2

theta = 120 degrees or 240 degrees


but this problem wants the negative angle


so theta = 120-360 or 240-360 = -240 or -120 degrees

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