Trigonometry and Complex Numbers - Youth Conway

= cos4x 1 = 2cos2 2x 2: Now, we can divide by 2 and expand the left side. ... cosx+ cosy= 2cos x+ y 2 cos x y 2 ; cosx cosy= 2sin x+ y 2 sin x y 2 ; sinx+ siny= 2sin x+ y 2 cos x y 2 ; sinx siny= 2sin x y 2 cos x+ y 2 : It is usually not necessary to memorize any of these identities for problems because ................
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