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Prove that: (cosalpha+cosbeta)^2+(s inal...

Prove that: `(cosalpha+cosbeta)^2+(s inalpha+s inbeta)^2=4cos^2\ \ ((alpha-beta)/2)`

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Given,`(cos alpha + cos beta)^2 + (sin alpha + sin beta)^2`
` = cos^2alpha + cos^2beta + 2 cos alpha cos beta + sin^2alpha + sin^2beta + 2 sin alpha sin beta`
`= (cos^2alpha + sin^2alpha) + (cos^2beta + cos^2beta) + 2(cos alpha cos beta + sin alpha sin beta)`
`= 1 + 1 + 2 cos(alpha – beta)`
...
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