Question

WHATS THE VALUE OF

$\cos(3 \: thita ) =$
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Answer 1

Answer : 4cos³A - 3cosA

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By formula :

cos3A = 4cos³A - 3cosA

By Solution :

cos3A = cos(2A+A)

As we know that :

As we know that : cos(A+B) = cosAcosB – sinAsinB

=> cos(2A+A) = cos2A.cosA-sin2AsinA

As we know that :

As we know that : cos2A = 2cos²A-1 and

As we know that : cos2A = 2cos²A-1 and sin2A = 2sinAcosA

=> cos(2A+A)= (2cos²A-1).cosA - (2sinAcosA).sinA

=> 2cos³A - cosA - 2sin²A.cosA

=> 2cos³A - cosA - 2(1-cos²A).cosA

=> 2cos³A - cosA - 2cosA + 2cos³A

=> 4cos³A - 3cosA