If $\frac { x }{ a } + \frac { y }{ b } =1$ and $\frac { { x }^{ 2 } }{ a } + \frac { { y }^{ 2 } }{ b } = \frac { ab }{ a+b }$ then the value of $\frac { { x }^{ n+1 } }{ a } + \frac { { y }^{ n+1 } }{ b }$ is:

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