If x^2 - 2 cosalpha x + 1 = 0 and y^2 - ...

If `x^2 - 2 cosalpha x + 1 = 0` and `y^2 - 2 cosbeta y + 1 = 0` then `2 cos(alpha + beta)` is equal to `(i)( x/y) + (y/x) (ii)(x+1/y)(y+1/x) (iii) xy+1/(xy) ` (iv) none of these

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