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If cosx+cosy-cos(x+y)=3/2, then x+y=0 (...

If `cosx+cosy-cos(x+y)=3/2,` then `x+y=0` (b) `x=2y` `x=y` (d) `2x=y`

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`sec^2theta=(4xy)/(x+y)^2`
`sec^2theta>=1`
`(4xy)/(x+y)^2>=1`
`(x+y)^2<=4xy`
`(x+y)^2-4xy<=0`
`(x-y)^2<=0`
For real values of x and y
`(x-y)^2>=0`
...
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