Consider the derivation of an alternate form of the cosine double angle identity.
What is the error in this derivation?
In step 1, cos(2x) is equal to cos2(x) + sin2(x).
In step 2, sin2(x) should have been replaced with 1 + cos2(x).
In step 3, cos2(x) – 1 – cos2(x) should be cos2(x) – 1 + cos2(x).
In step 4, 2cos2(x) – 1 should be 1 – 2cos2(x).
