3. For APPLICANTS IN $\left\{ \begin{array} { l } \text { MATHEMATICS } \\ \text { MATHEMATICS \& STATISTICS } \\ \text { MATHEMATICS \& PHILOSOPHY } \\ \text { MATHEMATICS \& COMPUTER SCIENCE } \end{array} \right\}$ ONLY.
Computer Science and Computer Science \& Philosophy applicants should turn to page 16.
In this question we shall investigate when functions are close approximations to each other. We define $| x |$ to be equal to $x$ if $x \geqslant 0$ and to $- x$ if $x < 0$. With this notation we say that a function $f$ is an excellent approximation to a function $g$ if
$$| f ( x ) - g ( x ) | \leqslant \frac { 1 } { 320 } \quad \text { whenever } \quad 0 \leqslant x \leqslant \frac { 1 } { 2 }$$
we say that $f$ is a good approximation to a function $g$ if
$$| f ( x ) - g ( x ) | \leqslant \frac { 1 } { 100 } \quad \text { whenever } \quad 0 \leqslant x \leqslant \frac { 1 } { 2 }$$
For example, any function $f$ is an excellent approximation to itself. If $f$ is an excellent approximation to $g$ then $f$ is certainly a good approximation to $g$, but the converse need not hold.
(i) Give an example of two functions $f$ and $g$ such that $f$ is a good approximation to $g$ but $f$ is not an excellent approximation to $g$.
(ii) Show that if
$$f ( x ) = x \quad \text { and } \quad g ( x ) = x + \frac { \sin \left( 4 x ^ { 2 } \right) } { 400 }$$
then $f$ is an excellent approximation to $g$. For the remainder of the question we are going to a try to find a good approximation to the exponential function. This function, which we shall call $h$, satisfies the following equation
$$h ( x ) = 1 + \int _ { 0 } ^ { x } h ( t ) \mathrm { d } t \quad \text { whenever } \quad x \geqslant 0$$
You may not use any other properties of the exponential function during this question, and any attempt to do so will receive no marks.
Let
$$f ( x ) = 1 + x + \frac { x ^ { 2 } } { 2 } + \frac { x ^ { 3 } } { 6 }$$
(iii) Show that if
$$g ( x ) = 1 + \int _ { 0 } ^ { x } f ( t ) \mathrm { d } t$$
then $f$ is an excellent approximation to $g$.
(iv) Show that for $x \geqslant 0$
$$h ( x ) - f ( x ) = g ( x ) - f ( x ) + \int _ { 0 } ^ { x } ( h ( t ) - f ( t ) ) \mathrm { d } t$$
(v) You are given that $h ( x ) - f ( x )$ has a maximum value on the interval $0 \leqslant x \leqslant 1 / 2$ at $x = x _ { 0 }$. Explain why
$$\int _ { 0 } ^ { x } ( h ( t ) - f ( t ) ) \mathrm { d } t \leqslant \frac { 1 } { 2 } \left( h \left( x _ { 0 } \right) - f \left( x _ { 0 } \right) \right) \quad \text { whenever } \quad 0 \leqslant x \leqslant \frac { 1 } { 2 }$$
(vi) You are also given that $f ( x ) \leqslant h ( x )$ for all $0 \leqslant x \leqslant \frac { 1 } { 2 }$. Show that $f$ is a good approximation to $h$ when $0 \leqslant x \leqslant \frac { 1 } { 2 }$.
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