`cos x*cos y` = `(1/2)*(cos(x+y) + cos(x-y))`
`cos(a+b)*cos(a-b)` = `(1/2)*(cos a + b + a - b) + cos(a + b - a + b))`
= `(1/2)*(cos 2a + cos 2b) `
As a sum `cos(a+b)*cos(a-b) = (1/2)*(cos 2a + cos 2b)`
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