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15.

If α,β \alpha, \beta and γ \gamma are the roots of the equation x3+px+q=0 \mathrm{x}^{3}+\mathrm{px}+\mathrm{q}=0 , then form the cubic equation whose roots are α+βγ2,β+γα2,α+γβ2 \frac{\alpha+\beta}{\gamma^{2}}, \frac{\beta+\gamma}{\alpha^{2}}, \frac{\alpha+\gamma}{\beta^{2}} .

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Given equation:
x3+px+q=0x^3 + px + q = 0
Since α,β,γ\alpha, \beta, \gamma are the roots of this equation, we have:
α+β+γ=0\alpha + \beta + \gamma = 0
αβ+βγ+γα=p\alpha\beta + \beta\gamma + \gamma\alpha = p
αβγ=−q\alpha\beta\gamma = -q

Now, the roots of the required equation are:
y1=α+βγ2=−γγ2=−1γy_1 = \frac{\alpha+\beta}{\gamma^2} = \frac{-\gamma}{\gamma^2} = -\frac{1}{\gamma}
y2=β+γα2=−αα2=−1αy_2 = \frac{\beta+\gamma}{\alpha^2} = \frac{-\alpha}{\alpha^2} = -\frac{1}{\alpha}
y3=α+γβ2=−ββ2=−1βy_3 = \frac{\alpha+\gamma}{\beta^2} = \frac{-\beta}{\beta^2} = -\frac{1}{\beta}

Let y=−1x  ⟹  x=−1yy = -\frac{1}{x} \implies x = -\frac{1}{y}.

Substituting x=−1yx = -\frac{1}{y} into the original equation x3+px+q=0x^3 + px + q = 0:
(−1y)3+p(−1y)+q=0\left(-\frac{1}{y}\right)^3 + p\left(-\frac{1}{y}\right) + q = 0
−1y3−py+q=0-\frac{1}{y^3} - \frac{p}{y} + q = 0

Multiplying throughout by −y3-y^3:
1+py2−qy3=01 + py^2 - qy^3 = 0
qy3−py2−1=0qy^3 - py^2 - 1 = 0

Therefore, the required cubic equation is:
qx3−px2−1=0qx^3 - px^2 - 1 = 0

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