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Explain how the sum and difference formula cos(A+/- B) = cosA cosB -/+ sinA sinB could be useful?

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You could find the exact value of cos of 15 degrees by using cos(45 - 30):-

cos(45 - 30) = cos 45cos30 + sin45 sin30 = 1 /sqrt2 * sqrt3/2 + 1/sqrt2* 1/2
User Daniel Dietrich
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