cosA= x then cos4A =?
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Answer:
Cos2A =2cos²A-1
Cos4A =2cos²2A-1
=2(cos2A)²-1
=2(2cos²A-1)²-1
=2(4cos²A+1-4cosA)-1
=8cos²A+2-8cosA-1
=8cos²A-8cos²A+1
=8X²-8X+1
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