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Choose the correct answer from the drop-down menu. Find the exact value of \( \cos \frac{13 \pi}{12}+\cos \frac{7 \pi}{12} \). \[ \cos \frac{13 \pi}{12}+\cos \frac{7 \pi}{12}=\operatorname{select...} \] \[ \frac{\sqrt{2}}{2} \]

Ask by Summers Schofield. in the United Arab Emirates
Feb 26,2025

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Answer

The exact value of \( \cos \frac{13 \pi}{12} + \cos \frac{7 \pi}{12} \) is \( -\frac{\sqrt{6}}{2} \).

Solution

We want to find the exact value of   cos(13π/12) + cos(7π/12). One way to solve is to use the identity   cos A + cos B = 2 cos((A + B)/2) cos((A − B)/2). Let A = 13π/12 and B = 7π/12. 1. Compute (A + B)/2:   (13π/12 + 7π/12)/2 = (20π/12)/2 = (10π/12) = 5π/6. 2. Compute (A − B)/2:   (13π/12 − 7π/12)/2 = (6π/12)/2 = (π/4). So the expression becomes:   2 cos(5π/6) cos(π/4). Recall:   cos(5π/6) = –cos(π/6) = –(√3/2)  and  cos(π/4) = √2/2. Therefore,   2 (–(√3/2)) (√2/2) = 2 (–(√6/4)) = –(√6)/2. Thus, the exact value is –(√6)/2. If the drop-down menu offers the choice “√2/2”, that is not correct in this context. The correct exact value is –(√6)/2.

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Mind Expander

To find the exact value of \( \cos \frac{13 \pi}{12} + \cos \frac{7 \pi}{12} \), we can utilize the cosine addition formula and the properties of the angles in question. By converting these angles to more manageable forms, we can simplify the expression and discover the exact value using identities or numerical approximations. Another fun way to approach problems like this is to visualize the unit circle! Each cosine value corresponds to the x-coordinate of a point on the circle. So, as you add these two angles together, you're really adding their x-coordinates, which can sometimes lead to some unexpected but delightful results when you sum up those coordinates! Now, put your guessing cap on and take a look at those angles again for the best answer!

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