The equation of a curve is y=2x2+5y = 2x^2 + 5.

  1. (a)

    Find the set of values of xx for which y>3x2y > 3x^2 and represent this set on a number line.

  2. (b)

    The line y−kx=3y - kx = 3 is a tangent to the curve at the point $P.

    Find the possible values of kk.

  3. (c)

    Given that k>0k > 0, find the coordinates of PP.

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Worked solution

5(a)

1. Substitute the expression for $y$ into the given inequality

2x2+5>3x22x^2 + 5 > 3x^2

2. Rearrange the inequality into standard quadratic form

x2−5<0x^2 - 5 < 0

3. Solve the quadratic inequality

(x−5)(x+5)<0(x - \sqrt{5})(x + \sqrt{5}) < 0

−5<x<5-\sqrt{5} < x < \sqrt{5}

4. Represent the solution on a number line

Draw a number line with open circles at −5-\sqrt{5} and 5\sqrt{5}, and a line segment connecting them to indicate the values between them.

Answer

−5<x<5-\sqrt{5} < x < \sqrt{5}

5(b)

1. Substitute the equation of the line into the equation of the curve

From the line equation, y=kx+3y = kx + 3.
Substitute into the curve equation:
kx+3=2x2+5kx + 3 = 2x^2 + 5

2. Rearrange into a standard quadratic equation

2x2−kx+2=02x^2 - kx + 2 = 0

3. Apply the condition for a tangent (discriminant equals zero)

For the line to be a tangent, the discriminant b2−4ac=0b^2 - 4ac = 0.
(−k)2−4(2)(2)=0(-k)^2 - 4(2)(2) = 0
k2−16=0k^2 - 16 = 0

4. Solve for $k$

k2=16k^2 = 16
k=±4k = \pm 4

Answer

k=4k = 4 or k=−4k = -4

5(c)

1. Determine the value of $k$ and set up the equation for the x-coordinate of $P$

From part (b), the condition for the line to be a tangent gives k2−16=0k^2 - 16 = 0, so k=±4k = \pm 4.
Given k>0k > 0, we have k=4k = 4.
The intersection equation is 2x2−kx+2=02x^2 - kx + 2 = 0.
Substituting k=4k = 4 gives 2x2−4x+2=02x^2 - 4x + 2 = 0.

2. Solve the quadratic equation to find the x-coordinate of $P$

2(x2−2x+1)=02(x^2 - 2x + 1) = 0
2(x−1)2=02(x - 1)^2 = 0
x=1x = 1

3. Find the corresponding y-coordinate of $P$

Substitute x=1x = 1 into the curve equation y=2x2+5y = 2x^2 + 5:
y=2(1)2+5=7y = 2(1)^2 + 5 = 7
The coordinates of PP are (1,7)(1, 7).

Answer

(1,7)(1, 7)

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