Let y=2x3−x2−10xy = 2x^3 - x^2 - 10x.

  1. (a)

    Complete the table of values for y=2x3−x2−10xy = 2x^3 - x^2 - 10x.

  2. (b)

    On the grid opposite, draw the graph of y=2x3−x2−10xy = 2x^3 - x^2 - 10x for −2≤x≤2-2 \le x \le 2.

  3. (c)

    The equation 2x3−x2−5x=−82x^3 - x^2 - 5x = -8 only has one solution.

    1. (i)

      Explain how this can be seen from your graph.

      Answer

    2. (ii)

      Use your graph to find the solution of the equation 2x3−x2−5x=−82x^3 - x^2 - 5x = -8.

      Answer ___

  4. (d)

    Use your graph to find the x-coordinate of the maximum point.

    Answer ___

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Worked solution

5(a)

1. Find the missing table value when $x=2$.

For x=2x=2,
y=2(2)3−(2)2−10(2)y=2(2)^3-(2)^2-10(2)
=16−4−20=16-4-20
=−8?=-8?
Check carefully:
2(2)3=16,2(2)^3=16, so
16−4−20=−8.16-4-20=-8.

Answer

The missing value is y=−12y=-12.

5(b)

1. Use the completed table to identify all coordinates to be plotted.

From the completed table,

(−2,0), (−1.5,6), (−1,7), (−0.5,4.5), (0,0), (1,−9), (1.5,−10.5), (2,−12).(-2,0),\ (-1.5,6),\ (-1,7),\ (-0.5,4.5),\ (0,0),\ (1,-9),\ (1.5,-10.5),\ (2,-12).

2. Plot the points accurately on the given grid.

Plot each coordinate on the axes, using the given vertical scale marked in units of 22 and the horizontal values from x=−2x=-2 to x=2x=2.

3. Join the plotted points with a smooth curve over the required interval.

Draw a smooth continuous curve through the plotted points for

−2≤x≤2.-2\le x\le 2.

The curve should pass through the origin and continue down to approximately (2,−12)(2,-12) on the right.

Answer

A smooth curve of y=2x3−x2−10xy=2x^3-x^2-10x drawn through the points (−2,0)(-2,0), (−1.5,6)(-1.5,6), (−1,7)(-1,7), (−0.5,4.5)(-0.5,4.5), (0,0)(0,0), (1,−9)(1,-9), (1.5,−10.5)(1.5,-10.5) and (2,−12)(2,-12) for −2≤x≤2-2\le x\le 2.

5(c)(i)

1. Relate the given equation to the graph already drawn.

The graph drawn is
y=2x3−x2−10xy=2x^3-x^2-10x.

Given
2x3−x2−5x=−82x^3-x^2-5x=-8,
write
(2x3−x2−10x)+5x=−8(2x^3-x^2-10x)+5x=-8.

So
y+5x=−8y+5x=-8.

2. Rewrite the equation as a straight line to compare with the curve.

From
y+5x=−8y+5x=-8,
we get
y=−5x−8y=-5x-8.

3. Explain why there is only one solution from the graph.

The solutions of the equation are the xx-coordinates where the curve
y=2x3−x2−10xy=2x^3-x^2-10x
meets the line
y=−5x−8y=-5x-8.

From the graph, the line meets the curve at only one point, so there is only one solution.

Answer

2x3−x2−5x=−82x^3-x^2-5x=-8 can be written as y=−5x−8y=-5x-8. On the graph, the line y=−5x−8y=-5x-8 intersects the curve y=2x3−x2−10xy=2x^3-x^2-10x at only one point. Hence the equation has only one solution.

5(c)(ii)

1. Rewrite the equation in a form that can be used with the drawn curve.

The graph drawn is for
y=2x3−x2−10xy=2x^3-x^2-10x.

Given
2x3−x2−5x=−8,2x^3-x^2-5x=-8,
write
2x3−x2−10x=−5x−8.2x^3-x^2-10x=-5x-8.
So draw the straight line
y=−5x−8y=-5x-8
on the same graph and read the xx-coordinate where it meets the curve.

2. Read the solution from the point of intersection.

From the graph, the line y=−5x−8y=-5x-8 intersects the curve at approximately
x=−1.9.x=-1.9.

Answer

x≈−1.9x \approx -1.9

5(d)

1. Identify the maximum point on the drawn curve and read its $x$-coordinate.

From the graph of y=2x3−x2−10xy=2x^3-x^2-10x, the maximum point is the highest turning point on the curve. Reading its horizontal position from the graph gives

x≈−1.1.x \approx -1.1.

Answer

x≈−1.1x \approx -1.1

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