Proving \(\displaystyle{\cos{{\left({w}_{{1}}{t}\right)}}}+{\cos{{\left({w}_{{2}}{t}\right)}}}={2}{\cos{{\left({\frac{{{1}}}{{{2}}}}{\left({w}_{{1}}+{w}_{{2}}\right)}{t}\right)}}}{\cos{{\left({\frac{{{1}}}{{{2}}}}{\left({w}_{{1}}-{w}_{{2}}\right)}{t}\right)}}}\)

cleffavw8

cleffavw8

Answered question

2022-03-29

Proving cos(w1t)+cos(w2t)=2cos(12(w1+w2)t)cos(12(w1w2)t)

Answer & Explanation

Lana Hamilton

Lana Hamilton

Beginner2022-03-30Added 12 answers

Remember that
cos(α+β)=cos(α)cos(β)sin(α)sin(β)
and
cos(αβ)=cos(α)cos(β)+sin(α)sin(β)
so you can add up both equalities to get
cos(α+β)+cos(αβ)=2cos(α)cos(β)
so now you want that α+β=ω1t and αβ=ω2t so you have to solve the system of equations
α+β=ω1t
αβ=ω2t
Which gives you
α=12(ω1t+ω2t)
β=12(ω1tω2t)
which is exactly what you wanted to prove

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