Prove that :- sin4 – cos4 = 1-2 cos2 .
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Answer:
Given:
cos
4
A−sin
4
A+1=(cos
2
A−sin
2
A)(cos
2
A+sin
2
A)+1
=cos
2
A−sin
2
A+1
=cos
2
A+cos
2
A
=2cos
2
A
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