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Find the real value of a for which 3i^3-...

Find the real value of a for which `3i^3-2a i^2+(1-a)i+5` is real.

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`3 i^3-2 a i^2+(1-a) i+5 `
`=-3 i+2 a+(1-a) i+5 `
`=(2 a+5)+i(1-a-3) `
`=(2 a+5)+i(-2-a)`

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