(cos^2 10°-cos^2 80°)+sin^2 20°
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(cos^210°-cos^280°)+sin^220°(cos^210°-cos^280°)+sin^220°(cos^210°-cos^280°)+sin^220°(cos^210°-cos^28
(cos^2 10°-cos^2 80°)+sin^2 20°
(cos^2 10°-cos^2 80°)+sin^2 20°
(cos^2 10°-cos^2 80°)+sin^2 20°
(cos^2 10°-cos^2 80°)²+sin^2 20°
=(cos^2 10°-sin^2 10°)²+sin^2 20°
=cos²20＋sin²20°
=1