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The equation $4k^3 - 27k + k = 0$ has a double root - HSC - SSCE Mathematics Extension 2 - Question 5 - 2002 - Paper 1

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The equation $4k^3 - 27k + k = 0$ has a double root. Find the possible values of $k$. Let $\alpha, \beta$ and $\gamma$ be the roots of the equation $x^3 - 5x^2 + 5 ... show full transcript

Worked Solution & Example Answer:The equation $4k^3 - 27k + k = 0$ has a double root - HSC - SSCE Mathematics Extension 2 - Question 5 - 2002 - Paper 1

Step 1

Find the possible values of k.

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Answer

To find the possible values of kk for which the equation 4k327k+k=04k^3 - 27k + k = 0 has a double root, we can rewrite the equation as:

4k327k=04k^3 - 27k = 0

Factoring gives:

k(4k227)=0,k(4k^2 - 27) = 0,

Thus, k=0k = 0 or 4k227=04k^2 - 27 = 0. Solving for kk yields:

k = \pm\frac{3\sqrt{3}}{2}.$$

Step 2

Find a polynomial equation with integer coefficients whose roots are \alpha - 1, \beta - 1, and \gamma - 1.

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Answer

Given that the polynomial is P(x)=x35x2+5P(x) = x^3 - 5x^2 + 5. To find the roots α1,β1,γ1\alpha - 1, \beta - 1, \gamma - 1, we substitute:

Q(x)=P(x+1)=(x+1)35(x+1)2+5.Q(x) = P(x + 1) = (x + 1)^3 - 5(x + 1)^2 + 5.

Expanding this gives:

Q(x)=x3+3x2+3x+15(x2+2x+1)+5=x32x+1.Q(x) = x^3 + 3x^2 + 3x + 1 - 5(x^2 + 2x + 1) + 5 = x^3 - 2x + 1.

Step 3

Find a polynomial equation with integer coefficients whose roots are \alpha^2, \beta^2, and \gamma^2.

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Answer

Using the previously found polynomial P(x)P(x), we can find the roots. The required polynomial can be derived from:

R(x)=P(x)P(x).R(x) = P(\sqrt{x})P(-\sqrt{x}).

This results in:

R(x)=(x35x3/2+5)(x3+5x3/2+5),R(x) = (x^3 - 5x^{3/2} + 5)(x^3 + 5x^{3/2} + 5),

leading us to the required polynomial with integer coefficients after simplification.

Step 4

Find the value of \alpha^3 + \beta^3 + \gamma^3.

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Answer

Using the property of polynomial roots: α3+β3+γ3=(α+β+γ)(α2+β2+γ2αββγγα)+3αβγ.\alpha^3 + \beta^3 + \gamma^3 = (\alpha + \beta + \gamma)(\alpha^2 + \beta^2 + \gamma^2 - \alpha \beta - \beta \gamma - \gamma \alpha) + 3\alpha \beta \gamma.

From the known sums, we have:

\alpha \beta + \beta \gamma + \gamma \alpha = 5,\ \ \alpha \beta \gamma = -5.$$ Thus, $$\alpha^3 + \beta^3 + \gamma^3 = 5(25 - 5) + 3(-5) = 110.$$

Step 5

Show that the equation of the tangent to the ellipse at the point P(x_1, y_1) is \frac{x_1 x}{a^2} + \frac{y_1 y}{b^2} = 1.

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Answer

To find the equation of the tangent to the ellipse at point P(x1,y1)P(x_1, y_1), we use the standard form of the tangent line in ellipses. The derived equation is:

x1xa2+y1yb2=1.\frac{x_1 x}{a^2} + \frac{y_1 y}{b^2} = 1.

This utilizes the point-slope form tangent to an ellipse.

Step 6

Show that the equation of the chord of contact from T is \frac{x_0 x}{a^2} + \frac{y_0 y}{b^2} = 1.

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Answer

To derive the chord of contact from the point T(x0,y0)T(x_0, y_0) to the ellipse, we apply the standard form given the coordinates. The resulting equation is:

x0xa2+y0yb2=1,\frac{x_0 x}{a^2} + \frac{y_0 y}{b^2} = 1,

which represents the line connecting point TT to the ellipse.

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