We can write cos (-x) = cos (0-x), such as the following
identity may now be used:
cos (a-b) = cos a*cos b + sin
a*sin b
Let a = 0 and b =
x
cos (0-x) = cos 0*cos x + sin 0*sin
x
But cos 0 = 1 and sin 0 =
0.
cos (0 - x) = 1*cos x + 0*sin
x
cos (0 - x) = cos
x
Therefore, the identity cos(-x) = cos x is
verified.
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