If E = cos 2 ⁡ 71 o + cos 2 ⁡ 49 o + cos ⁡ 71 o cos ⁡ 49 o , then the value of 10 E is equal to

Question

If E = cos 2 ⁡ 71 o + cos 2 ⁡ 49 o + cos ⁡ 71 o cos ⁡ 49 o , then the value of 10 E is equal to, 7 . 5, 2 . 5, 12 . 5, 10 . 5

MARKS App · Accepted Answer

E = 1 + cos ⁡ 142 o 2 + 1 + cos ⁡ 98 o 2 + 1 2 2 cos ⁡ 71 o cos ⁡ 49 o = 1 + 1 2 cos ⁡ 142 o + cos ⁡ 98 o + 1 2 cos ⁡ 120 o + cos ⁡ 22 o = 1 + 1 2 2 cos ⁡ 120 o cos ⁡ 22 o + 1 2 - 1 2 + cos ⁡ 22 o = 1 + - 1 2 cos ⁡ 22 o - 1 4 + 1 2 cos ⁡ 22 o = 3 4 E = 3 4 = 0.75 ⇒ 10 E = 7.5

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