please prove this asap

Dear Student,

w e k n o w, C o s 2 ϕ = 2 C o s^{2} ϕ - 1 2 C o s^{2} ϕ = 1 + C o s 2 ϕ n o w p u t t i n g ϕ = \frac{ϕ}{2}, w e g e t 2 C o s^{2} \frac{ϕ}{2} = 1 + C o s 2 \frac{ϕ}{2} = 1 + C o s ϕ

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