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Ex 4.2
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Solutions
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(a) The shape resembles the graph in the top left section of the table on page 101.
\begin{align} \text{When } & x = 0, \\ y & = 3(0)^5 \\ & = 0 \\ \\ x \text{ and } y & \text{-intercepts are } (0, 0) \end{align}
(b) The shape resembles the graph in the top right section of the table on page 101.
\begin{align} \text{When } & x = 0, \\ y & = -3(0)^4 \\ & = 0 \\ \\ x \text{ and } y & \text{-intercepts are } (0, 0) \end{align}
(c) The shape resembles the graph in the bottom left section of the table on page 101.
\begin{align} y & = {2 \over x^3} \\ & = 2 \left(1 \over x^3\right) \\ & = 2x^{-3} \end{align}
(d) The shape resembles the graph in the bottom right section of the table on page 101.
\begin{align} y & = -{2 \over x^4} \\ & = -2 \left(1 \over x^4\right) \\ & = -2x^{-4} \end{align}
(a) The shape resembles the graph in the first row of the table on page 103.
\begin{align} \text{When } & x = 0, \\ y & = (0)^{1 \over 4} \\ & = 0 \\ \\ x \text{ and } y & \text{-intercepts are } (0, 0) \end{align}
(b) The shape resembles the graph in the first row of the table on page 103.
\begin{align} \text{When } & x = 0, \\ y & = (0)^{2 \over 3} \\ & = 0 \\ \\ x \text{ and } y & \text{-intercepts are } (0, 0) \end{align}
(c) The shape resembles the graph in the second row of the table on page 103.
\begin{align} \text{When } & x = 0, \\ y & = (0)^{3 \over 2} \\ & = 0 \\ \\ x \text{ and } y & \text{-intercepts are } (0, 0) \end{align}
(d) The shape resembles the graph in the second row of the table on page 103.
\begin{align} \text{When } & x = 0, \\ y & = 3(0)^{3 \over 2} \\ & = 0 \\ \\ x \text{ and } y & \text{-intercepts are } (0, 0) \end{align}
(e) The shape resembles the graph in the second row of the table on page 103.
\begin{align} \text{When } & x = 0, \\ y & = -3(0)^{3 \over 2} \\ & = 0 \\ \\ x \text{ and } y & \text{-intercepts are } (0, 0) \end{align}
(a) The shape resembles the graph in the third row of the table on page 103.
(b) The shape resembles the graph in the third row of the table on page 103.
(c) The shape resembles the graph in the third row of the table on page 103.
(a) The shape resembles the graph in the top right section of the table on page 101. Note radius of the sphere cannot be negative.
(b) The shape resembles the graph in the top left section of the table on page 101. Note radius of the sphere cannot be negative.
(c)
\begin{align} \text{Area of circle} & = \pi \times \text{Radius} \times \text{Radius} \\ x & = \pi \times y \times y \\ x & = \pi y^2 \\ {x \over \pi} & = y^2 \\ \pm \sqrt{x \over \pi} & = y \\ \pm {\sqrt{x} \over \sqrt{\pi}} & = y \\ \pm {1 \over \sqrt{\pi}} \sqrt{x} & = y \\ \pm {1 \over \sqrt{\pi}} x^{1 \over 2} & = y \\ \\ \text{Since radius of } & \text{cannot be negative,} \\ \\ \therefore y & = {1 \over \sqrt{\pi}} x^{1 \over 2} \end{align}
(The shape resembles the graph in the second row of the table on page 103)
(i)
\begin{align} y & = 2\pi \sqrt{x \over 10} \\ & = 2 \pi {\sqrt{x} \over \sqrt{10}} \\ & = {2\pi \over \sqrt{10}} \sqrt{x} \\ & = {2\pi \over \sqrt{10}} x^{1 \over 2} \end{align}
(ii)
\begin{align} y & = {2\pi \over \sqrt{10}} x^{1 \over 2} \\ \\ \text{When } & x = 40, \\ y & = {2\pi \over \sqrt{10}} (40)^{1 \over 2} \\ & = 4\pi \end{align}
(iii)
\begin{align} y & = {2\pi \over \sqrt{10}} x^{1 \over 2} \\ \\ \text{When } & y = \pi, \\ \pi & = {2\pi \over \sqrt{10}} x^{1 \over 2} \\ {\pi \sqrt{10} \over 2\pi} & = x^{1 \over 2} \\ {\sqrt{10} \over 2} & = x^{1 \over 2} \\ \left(\sqrt{10} \over 2\right)^2 & = x \\ 2.5 & = x \end{align}
(iv)
\begin{align} y & = {2\pi \over \sqrt{10}} x^{1 \over 2} \end{align}
(The shape resembles the graph in the first row of the table on page 103)
The shape of both graphs can be found in the first row of the table on page 103
$$ \text{The graphs are reflections of each other about the } x \text{-axis}$$
\begin{align} y & = {2 \over \sqrt{x}} \\ & = 2 \left(1 \over \sqrt{x}\right) \\ & = 2x^{-{1 \over 2}} \end{align}
(The shape resembles the graph in the third row of the table on page 103)
\begin{align} y & = 2x - 3 \phantom{000000} [\text{Straight line } y = mx + c] \\ \\ \text{When } & x = 0, \\ y & = 2(0) - 3 \\ & = -3 \\ \\ \implies y & \text{-intercept is } - 3 \\ \\ \text{When } & y = 0, \\ 0 & = 2x - 3 \\ 3 & = 2x \\ {3 \over 2} & = x \\ 1.5 & = x \\ \\ \implies x & \text{-intercept is } 1.5 \end{align}
$$ \therefore 1 \text{ solution} $$
The power function resembles the graph in the first row of the table on page 103
\begin{align} y & = 4 - x^2 \\ & = -x^2 + 4 \\ \\ \text{Since coefficient of } x^2 & \text{ is negative,} \text{ this is a maximum curve } (\cap) \\ \\ \text{When } & y = 0, \\ 0 & = -x^2 + 4 \\ x^2 & = 4 \\ x & = \pm \sqrt{4} \\ & = \pm 2 \\ \\ \implies x& \text{-intercepts} \text{ are } 2, -2 \\ \\ \text{When } x & = 0, \\ y & = -(0)^2 + 4 \\ & = 4 \\ \\ \implies y & \text{-intercept} \text{ is 4} \\ \\ x-\text{coordinate of turning point} & = {2 + (-2) \over 4} \\ & = 0 \\ \\ \text{When } & x = 0, \\ y & = -(0)^2 + 4 \\ & = 4 \\ \\ \implies & \text{Turning point}\text{ is (0, 4)} \end{align}
(Since $x \ge 0$, only the right side of $y = 4 - x^2$ is required.)
\begin{align} 4x^{1 \over 3} + x^2 & = 4 \\ \underbrace{4x^{1 \over 3}}_\text{Power function} & = \underbrace{4 - x^2}_\text{Quadratic function} \\ \\ \text{From the graph, } & \text{there is 1 point of intersection.} \\ \\ \therefore & \text{1 solution} \end{align}
(a)
\begin{align} y & = \sqrt{6x} \\ & = \sqrt{6} \sqrt{x} \\ & = \sqrt{6} x^{1 \over 2} \end{align}
(The shape resembles the graph in the first row of the table on page 103.)
(a)
\begin{align} y & = -\sqrt[3]{8x} \\ & = - \sqrt[3]{8} \sqrt[3]{x} \\ & = - 2 x^{1 \over 3} \end{align}
(The shape resembles the graph in the first row of the table on page 103.)
(a)
\begin{align} y & = {4 \over \sqrt[5]{32x^2}} \\ & = {4 \over \sqrt[5]{32} \sqrt[5]{x^2}} \\ & = {4 \over 2 (x^2)^{1 \over 5}} \\ & = {4 \over 2 x^{2 \over 5}} \\ & = {2 \over x^{2 \over 5}} \\ & = 2 \left(1 \over x^{2 \over 5}\right) \\ & = 2x^{-{2 \over 5}} \end{align}
(The shape resembles the graph in the third row of the table on page 103.)
(i)
\begin{align} y & = 2x + 1 \phantom{000} \text{ --- (1)} \\ \\ y & = {1 \over x} \phantom{000} \text{ --- (2)} \\ \\ \text{Substitute } & \text{(1) into (2),} \\ 2x + 1 & = {1 \over x} \\ 2x^2 + x & = 1 \\ 2x^2 + x - 1 & = 0 \\ (2x - 1)(x + 1) & = 0 \\ \\ 2x - 1 = 0 \phantom{000}&\text{or}\phantom{000} x + 1 = 0 \\ 2x = 1 \phantom{000}&\phantom{or000+1} x = - 1 \\ x = 0.5 \phantom{.0} & \\ \\ \text{Substitute } & \text{into (2),} \\ y = {1 \over 0.5} \phantom{0} & \phantom{or000+1} y = {1 \over -1} \\ = 2 \phantom{000} & \phantom{or000+1y} = -1 \end{align}
(ii)
\begin{align} y & = {1 \over x} \\ & = x^{-1} \end{align}
(The shape resembles the graph in the bottom left section of the table on page 101.)
\begin{align} y & = 2x + 1 \phantom{000000} [\text{Straight line } y = mx + c] \\ \\ \text{When } & x = 0, \\ y & = 2(0) + 1 \\ & = 1 \\ \\ \implies & y\text{-intercept is } 1 \\ \\ \text{When } & y = 0, \\ 0 & = 2x + 1 \\ -2x & = 1 \\ x & = {1 \over -2} \\ & = -0.5 \\ \\ \implies & x\text{-intercept is } -0.5 \end{align}
From part (i), the points of intersection betwen the two graphs are $(-1, -1)$ and $(0.5, 2)$.
(i)
\begin{align} 5x^{1 \over 2} & = 2x^{1 \over 6} \\ (5x^{1 \over 2})^6 & = (2x^{1 \over 6})^6 \\ (5)^6 (x^{1 \over 2})^6 & = (2)^6 (x^{1 \over 6})^6 \\ 15625 x^3 & = 64x \\ 15626 x^3 - 64x & = 0 \\ x(15626 x^2 - 64) & = 0 \\ \\ x = 0 \phantom{000}&\text{or}\phantom{000} 15626x^2 - 64 = 0 \\ & \phantom{or000-64} 15626x^2 = 64 \\ & \phantom{or000-6415626} x^2 = {64 \over 15626} \\ & \phantom{or000-64156260} x = \pm \sqrt{64 \over 15626} \\ & \phantom{or000-64156260x} = {8 \over 125} \text{ or } -{8 \over 125} \text{ (Reject)} \end{align}
Note $x = -{8 \over 125}$ is rejected as $\left(-{8 \over 125}\right)^{1 \over 2} $ has no real solutions.
(ii) The shape of both graphs resembles the graph in the first row of the table on page 103.
From part (i), the $x$-coordinates of the points of intersection are $0$ and ${8 \over 125}$
\begin{align} \text{When } & x = 0, \\ y & = 5(0)^{1 \over 2} \\ & = 0 \\ \\ \text{When } & x = {8 \over 125}, \\ y & = 5 \left(8 \over 125\right)^{1 \over 2} \\ & = 5 \sqrt{8 \over 125} \\ & = 5 \left( {\sqrt{8} \over \sqrt{125}} \right) \\ & = 5 \left( {\sqrt{4} \sqrt{2} \over \sqrt{25} \sqrt{5} } \right) \\ & = 5 \left( 2 \sqrt{2} \over 5 \sqrt{5} \right) \\ & = {2 \sqrt{2} \over \sqrt{5}} \end{align}
(i)
\begin{align} I & = \sqrt{2P \over L} \\ \\ \text{When } & L = 8, \\ I & = \sqrt{2P \over 8} \\ & = \sqrt{P \over 4} \\ & = {\sqrt{P} \over \sqrt{4}} \\ & = {P^{1 \over 2} \over 2} \\ & = {1 \over 2} P^{1 \over 2} \end{align}
(The shape resembles the graph in the first row of the table on page 103.)
(ii)
\begin{align} I & = \sqrt{2P \over L} \\ \\ \text{When } & P = 18, \\ I & = \sqrt{2(18) \over L} \\ & = \sqrt{36 \over L} \\ & = {\sqrt{36} \over \sqrt{L}} \\ & = {6 \over L^{1 \over 2}} \\ & = 6 \left(1 \over L^{1 \over 2}\right) \\ & = 6 L^{-{1 \over 2}} \end{align}
(The shape resembles the graph in the third row of the table on page 103.)
(iii)
\begin{align} I & = \sqrt{2P \over L} \\ \\ \text{When } & P = 18, \\ I & = \sqrt{2(18) \over L} \\ I & = \sqrt{36 \over L} \\ I & = {\sqrt{36} \over \sqrt{L}} \\ I & = {6 \over \sqrt{L}} \\ I \sqrt{L} & = 6 \\ \sqrt{L} & = {6 \over I} \\ (\sqrt{L})^2 & = \left(6 \over I\right)^2 \\ L & = {36 \over I^2} \\ L & = 36 \left(1 \over I^2\right) \\ L & = 36 I^{-2} \end{align}
(The shape resembles the graph in the bottom right section of the table on page 101.)
(i)
\begin{align} y & = {3 \over x} \\ & = 3 \left(1 \over x\right) \\ & = 3 x^{-1} \end{align}
(The shape resembles the graph in the bottom left section of the table on page 101.)
$$ y = 4x^{1 \over 5} $$
(The shape resembles the graph in the first row of the table on page 103.)
(ii)
\begin{align} & \text{From (i), graph meets at one point} \\ \\ & \therefore \text{Only 1 value of } x \end{align}
(i)
x | 0.5 | 1.0 | 1.5 | 2.0 | 2.5 | 3.0 | 3.5 | 4.0 |
y | -5.66 | -4 | -3.27 | -2.83 | -2.53 | -2.31 | -2.14 | -2 |
(ii)
x | 0.5 | 2.0 | 4.0 |
y | 0.5 | -1 | -3 |
(iii)
\begin{align} (1 - x)\sqrt{x} & = -4 \\ \\ \underbrace{1 - x}_\text{Graph from part (ii)} & = \underbrace{-{4 \over \sqrt{x}}}_\text{Graph from part (i)} \\ \\ \therefore & \text{ 1 solution} \end{align}
(i)
$$ y = 4x^{-{1 \over 2}} $$
(The shape resembles the graph in the third row of the table on page 103.)
\begin{align} y & = 2x \phantom{00000} [\text{Straight line}] \\ \\ \text{When } & x = 0, \\ y & = 0 \\ \\ \implies & x \text{ and } y\text{-intercept are } (0, 0) \end{align}
(ii)
\begin{align} \underbrace{2x}_\text{Line} & = \underbrace{4x^{-{1 \over 2}}}_\text{Power function} \\ \\ \therefore & \text{ 1 solution} \end{align}
(iii)
\begin{align} 2x & = 4x^{-{1 \over 2}} \\ {x \over x^{-{1 \over 2}}} & = {4 \over 2} \\ x^{1} \div x^{-{1 \over 2}} & = 2 \\ x^{1 - (-{1 \over 2})} & = 2 \\ x^{3 \over 2} & = 2 \\ \left( x^{3 \over 2} \right)^{2 \over 3} & = 2^{2 \over 3} \\ x & = 2^{2 \over 3} \\ \\ \text{Substitute } & x = 2^{2 \over 3} \text{ into } y = 2x, \\ y & = 2 \left(2^{2 \over 3}\right) \\ & = 2^1 \times 2^{2 \over 3} \\ & = 2^{1 + {2 \over 3}} \\ & = 2^{5 \over 3} \\ \\ \therefore \text{Point of} & \text{ intersection is } \left( 2^{2 \over 3}, 2^{5 \over 3} \right) \end{align}
(i)
\begin{align} 2x - 3\sqrt{x} - 2 & = 0 \\ 2(\sqrt{x})^2 - 3\sqrt{x} - 2 & = 0 \\ (2\sqrt{x} + 1)(\sqrt{x} - 2) & = 0 \\ \\ 2\sqrt{x} + 1 = 0 \phantom{000000}&\text{or}\phantom{000} \sqrt{x} - 2 = 0 \\ 2\sqrt{x} = - 1 \phantom{0000(} &\phantom{or000-2} \sqrt{x} = 2 \\ \sqrt{x} = -{1 \over 2} \phantom{0000} &\phantom{or000-20.} x = (2)^2 \\ x = \left(-{1 \over 2}\right)^2 \phantom{.} &\phantom{or000-20.} x = 4 \\ x = {1 \over 4} \phantom{00000.} & \\ \\ \text{Substitute } x = {1 \over 4} & \text{ into the original equation,} \\ \text{L.H.S} & = 2\left({1 \over 4}\right) - 3 \sqrt{{1 \over 4}} - 2 \\ & = -3 \\ & \ne \text{R.H.S} \\ \\ \therefore x \ne {1 \over 4} \\ \\ \text{Substitute } x = 4 & \text{ into the original equation,} \\ \text{L.H.S} & = 2(4) - 3\sqrt{4} - 2 \\ & = 0 \\ & = \text{R.H.S} \\ \\ \therefore x & = 4 \end{align}
(ii)
\begin{align} y & = 3\sqrt{x} \\ & = 3x^{1 \over 2} \end{align}
(The shape resembles the graph in the first row of the table on page 103.)
\begin{align} y & = 2x - 2 \phantom{000000} [\text{Straight line } y = mx + c] \\ \\ \text{When } & x = 0, \\ y & = 2(0) - 2 \\ & = - 2 \\ \\ \implies & y\text{-intercept is } -2 \\ \\ \text{When } & y = 0, \\ 0 & = 2x - 2 \\ -2x & = -2 \\ x & = {-2 \over -2} \\ & = 1 \\ \\ \implies & x\text{-intercept is } 1 \end{align}
\begin{align} y & = 2x - 2 \phantom{000} \text{ --- (1)} \\ \\ y & = 3\sqrt{x} \phantom{000} \text{ --- (2)} \\ \\ \text{Substitute } & \text{(1) into (2),} \\ 2x - 2 & = 3\sqrt{x} \\ 2x - 3\sqrt{x} - 2 & = 0 \\ \\ \text{From part } & \text{(i),} \\ x & = 4 \\ \\ \text{Substitute } & x = 4 \text{ into (1),} \\ y & = 2(4) - 2 \\ & = 6 \\ \\ \implies & \text{Point of intersection is } (4, 6) \end{align}
(i)
\begin{align} y & = a \sqrt[3]{x} \\ \\ \text{Using } & (8,1 ), \\ 1 & = a \sqrt[3]{8} \\ 1 & = a(2) \\ 1 & = 2a \\ {1 \over 2} & = a \\ \\ \\ y & = {b \over 2x} \\ \\ \text{Using } & (8, 1), \\ 1 & = {b \over 2(8)} \\ 1 & = {b \over 16} \\ 16 & = b \\ \\ \\ \therefore a & = {1 \over 2}, b = 16 \end{align}
(ii)
\begin{align} y & = {1 \over 2} \sqrt[3]{x} \\ y & = {1 \over 2}x^{1 \over 3} \\ \\ \\ y & = {16 \over 2x} \\ y & = {8 \over x} \\ y & = 8x^{-1} \end{align}
$$ y = 8x^{-1} \text{ is symmetrical about } y = x $$
(iii)
$$(-8, -1)$$