1 - 8 cos²α + 8 cos^4 α
= 1 – 8 cos²α ( 1 – cos²α )
= 1 – 8 cos²α sin²α
= 1 – 2 × ( 2sinαcosα)²
= 1 – 2 sin² 2α
= cos 2 (2α) [ as cos 2x = 1 – 2 sin²x]
= cos 4α
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1 - 8 cos²α + 8 cos^4 α
= 1 – 8 cos²α ( 1 – cos²α )
= 1 – 8 cos²α sin²α
= 1 – 2 × ( 2sinαcosα)²
= 1 – 2 sin² 2α
= cos 2 (2α) [ as cos 2x = 1 – 2 sin²x]
= cos 4α