How do I show that cos(z(adsbygoogle = window.adsbygoogle || []).push({}); _{1}+ z_{2}) = cos(z_{1})cos(z_{2}) - sin(z_{1})sin(z_{2})

I let z_{1}= a + ib & z_{2}= c + id

So cos(z_{1}+ z_{2}) = cos(a+ib+c+id) = cos(a+ib)cos(c+id) - sin(a+ib)sin(c+id) = cos(z_{1})cos(z_{2}) - sin(z_{1})sin(z_{2}). (From the trig identity).

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# How do I show that cos(z1 + z2) = cos(z1)cos(z2) - sin(z1)sin(z2)

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