Cos 130 - cos 110 = p Cos 20 = ?

TriGonoMetRi

cos 130° - cos 110° = p
-2 sin ((130 + 110)/2) sin ((130 - 110)/2) = p
-2 sin 120° sin 10° = p
-2 . 1/2 √3 . sin 10° = p
sin 10° = -p/√3

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cos 20°
= 1 - 2 sin² 10°
= 1 - 2(-p/√3)²
= 1 - 2/3 p²