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Prove that: `cos5A=16cos^5A-20cos^3A+5cosA` |
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Answer» `cos5A = cos(3A+2A) = cos3Acos2A - sin3Asin2A` `=(4cos^3A-3cosA)(2cos^2A -1) - (3sinA - 4sin^3A)(2sinAcosA)` `=(8cos^5A-10cos^3A+3cosA)- (3-4sin^2A)(2sin^2AcosA)` `=(8cos^5A-10cos^3A+3cosA)- (3-4(1-cos^2A))(2(1-cos^2A)cosA)` `=(8cos^5A-10cos^3A+3cosA)- (3-4+4cos^2A)(2cosA - 2cos^3A)` `=(8cos^5A-10cos^3A+3cosA)- (-8cos^5A+10cos^3A- 2cosA)` `=16cos^5A-20cos^3A+5cosA= R.H.S.` |
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