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Prove that cos(x+y)=cosxcosy-sinxsiny...

Prove that `cos(x+y)=cosxcosy-sinxsiny`

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Acc to the figure
`AC^2 = sqrt((1 - cos(x+y))^2 + ( 0 - sin(x+y)^2)) `
`= sqrt(1+ cos^2 (x+y) - 2cos(x+y) + sin^2(x+y))`
`= sqrt(2-2cos(x+y))`
`AC^2 = 2-2cos(x+y)`
`BD^2 = (cos x - cos y)^2 + (sin x + sin y)^2`
`cos^2 x + cos^2y- 2cosx cos y + sin^2 x + sin^2 y + 2 sinx siny`
`= 2 + 2sinx sin y - 2cos xcos y`
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