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secondary 3 | A Maths
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sorry i need help with this, thank you!
= cos (2A + A)
= cos2A cosA - sin2A sinA
= (2cos²A - 1)(cosA) - 2sinAcosA(sinA)
= 2cos³A - cosA - 2sin²A cosA
= 2cos³A - cosA - 2(1 - cos²A)(cosA)
= 2 cos³A - cosA - 2cosA + 2cos³A
= 4 cos³A - 3 cosA (shown)
cos⁴A - sin⁴A
= (cos²A)² - (sin²A)²
= (cos²A + sin²A)(cos²A - sin²A)
= (1) cos2A
= cos2A (shown)
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