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Question
Given that `sqrt(2)` is a zero of the cubic polynomial `6x^3 + sqrt(2)x^2 - 10x - 4sqrt(2)`, find its other two zeroes.
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Solution
Given, `sqrt(2)` is one of the zero of the cubic polynomial.
Then, `(x - sqrt(2))` is one of the factor of the given polynomial p(x) = `6x^3 + sqrt(2)x^2 - 10x - 4sqrt(2)`.
So, by dividing p(x) by `x - sqrt(2)`
`6x^2 + 7sqrt(2)x + 4`
`(x - sqrt(2))")"overline(6x^3 + sqrt(2)x^2 - 10x - 4sqrt(2))`
`6x^3 - 6sqrt(2)x^2`
– +
`7sqrt(2)x^2 - 10x - 4sqrt(2)`
`7sqrt(2)x^2 - 14x`
– +
`4x - 4sqrt(2)`
`4x - 4sqrt(2)`
0
`6x^3 + sqrt(2)x^2 - 10x - 4sqrt(2) = (x - sqrt(2)) (6x^2 + 7sqrt(2)x + 4)`
By splitting the middle term,
We get,
`(x - sqrt(2)) (6x^2 + 4sqrt(2)x + 3sqrt(2)x + 4)`
= `(x - sqrt(2)) [2x(3x + 2sqrt(2)) + sqrt(2)(3x + 2sqrt(2))]`
= `(x - sqrt(2)) (2x + sqrt(2)) (3x + 2sqrt(2))`
To get the zeroes of p(x),
Substitute p(x) = 0
`(x - sqrt(2)) (2x + sqrt(2)) (3x + 2sqrt(2))` = 0
`x = sqrt(2) , x = -sqrt(2)/2, x = (-2sqrt(2))/3`
Hence, the other two zeroes of p(x) are `-sqrt(2)/2` and `(-2sqrt(2))/3`.
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