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Last October 9, 2012, we did the derivation of the Sum and Difference of Two Angles Formula. Right now, we will derive the formulas for Double Angle Formula from the formulas of the Sum and Difference of Two Angles Formula.
Let's consider this one,
if θ = ϕ, then
Let's try this one,
if θ = ϕ, then
And finally, this one,
if θ = ϕ, then
