How To – The Culture SG

Jun222017 by KS TengNo Comments

Thinking [email protected] #7

<blockquote>[email protected] is a series of questions that we, as tutors feel that are useful in helping students think and improve their understanding.

Thinking [email protected] is curated by KS. More of him can be found <a href=”https://theculture.sg/our-team/tutor-ks/”>here</a>.</blockquote>

This is an application question for hypothesis testing from the 9758 H2 Mathematics Specimen Paper 2 Question 10.

The average time required for the manufacture of a certain type of electronic control panel is 17 hours. An alternative manufacturing process is trialled, and the time taken, $t$ hours, for the manufacture of each of 50 randomly chosen panels using the alternative process, in hours, is recorded. The results are summarized as follows

$n = 50$
$\sum t = 835.7$
$\sum t^2 = 14067.17$

The Production Manager wishes to test whether the average time taken for the manufacture of a control panel is different using the alternative process, by carrying out a hypothesis test.
(i) Explain whether the Production Manager should use a 1-tail or a 2-tail test.
(ii) Explain why the Production Manager is able to carry out a hypothesis test without knowing anything about the distribution of the times taken to manufacture the control panels.
(iii) Find unbiased estimates of the population mean and variance, and carry out the test at the 10% level of significance for the Production Manager.
(iv) Suggest a reason why the Production Manager might be prepared to use an alternative process that takes a longer average time than the original process.
The Finance Manager wishes to test whether the average time taken for the manufacture of a control panel is shorter using the alternative process. The Finance Manger finds that the average time taken for the manufacture of each of the 40 randomly chosen control panels, using the alternative process, is 16.7 hours. He carries out a hypothesis test at 10% level of significance.
(v) Explain, with justification, how the population variance of the times will affect the conclusion made by the Finance Manager.

Solution

Jun162017 by KS TengNo Comments

Thinking [email protected] #6

JC Mathematics, Mathematics

<blockquote>[email protected] is a series of questions that we, as tutors feel that are useful in helping students think and improve their understanding.

Thinking [email protected] is curated by KS. More of him can be found <a href=”https://theculture.sg/our-team/tutor-ks/”>here</a>.</blockquote>

This is a very interesting vectors question from a recent JC BT

Shown in the diagram is a methane molecule consisting of a carbon atom, G, with four hydrogen atoms, A, B, C, and D, symmetrically placed around it in three dimensions, such that the four hydrogen atoms form the vertices of a regular tetrahedron.

Treat A, B, C, D, and G as points. The coordinates of A, B, C, and D are given by (5, -2, 5), (5, 4, -1), (-1, -2, -1) and (-1, 4, 5) respectively, By considering the line DG and the symmetrical properties of methane, find the bond angle of methane, that is, $\angle DGA$ .

Answer

May272017 by KS TengNo Comments

Thinking [email protected] #4

JC Mathematics, Mathematics

[email protected] is a series of questions that we, as tutors feel that are useful in helping students think and improve their understanding.

Thinking [email protected] is curated by KS. More of him can be found here.

This is a question from 1993 Paper 1.

The positive integers, starting at 1, are grouped into sets containing $1, 2, 4, 8, \ldots$ integers, as indicated below, so that the number of integers in each set after the first is twice the number of integers in the previous set.

$\{ 1 \}, \{ 2, 3 \}, \{ 4, 5, 6, 7 \}, \{ 8, 9, 10, 11, 12, 13, 14, 15 \}, \ldots$

(i) Write down the expressions, in terms of $r$ for

(a) the number of integers in the $r^{th}$ set,

(b) the first integer in the $r^{th}$ set,

(ii) Given that the integer $1,000,000$ occurs in the $r^{th}$ set, find the integer value of $r$ .

(iii) The sum of all the integers in the $20^{th}$ set is denoted by $S$ , and the sum of all the integers in all of the first $20$ sets is denoted by $T$ . Show that $S$ may be expressed as $2^{18}(3 \times 2^{19} - 1)$ .

Hence, evaluate $\frac{T}{S}$ , correct to 4 decimal places.

Answer

May182017 by KS TengNo Comments

Thinking [email protected] #3

JC Mathematics, Mathematics

[email protected] is a series of questions that we, as tutors feel that are useful in helping students think and improve their understanding.

Thinking [email protected] is curated by KS. More of him can be found here.

This is a question from 1976 A’levels Paper 2. I thought it is pretty interesting to discuss the question with a little extension.

(a) In how many ways can 5 copies of a book be distributed among 10 people, if no-one gets more than one copy?

(b) In how many ways can 5 different books be distributed among 10 people if each person can get any number of books?

Solution

So now, let us modify it a bit.

Notice that the difference between (b) and (c) is that the book distributed is not identical. So for (c), we are pretty much distributing $r$ identical balls to $n$ distinct boxes. Whereas for (b) , we are pretty much distributing $r$ distinct balls to $n$ distinct boxes.

Apr262017 by KS TengNo Comments

Thinking [email protected] #2

JC Mathematics, Mathematics

[email protected] is a series of questions that we, as tutors feel that are useful in helping students think and improve their understanding.

Thinking [email protected] is curated by KS. More of him can be found here.

(i) Find the two possible values of $z$ such that $z^2 = 1 + \sqrt{3}i$ , leaving your answer in exact form $a + bi$ , where $a$ and $b$ are real numbers.

(ii) Hence or otherwise, find the exact roots of the equation

$2w^2 + 2 \sqrt{6}w + 1 - 2 \sqrt{3} i = 0$

Solution

Apr242017 by KS TengNo Comments

Thinking [email protected] #1

JC Mathematics, Mathematics

[email protected] is a series of questions that we, as tutors feel that are useful in helping students think and improve their understanding.

Thinking [email protected] is curated by KS. More of him can be found here.

Each card in a deck of cards bear a single number from 1 to 5 such that there are $n$ cards bearing the number $n$ , where $n = 1, 2, 3, 4, 5$ . One card is randomly drawn from the deck. Let $X$ be the number on the card drawn.

(i) Find the probability distribution of $X$ .

(ii) Show that $\mathbb{E}(X) = \frac{11}{3}$ and find $\text{Var}(X)$ .

Andrew draws one card from the deck, notes the number and replaces it. The deck is shuffled and Beth also draws on card from the deck and notes the number. Andrew’s score is $k$ times the number on teh card he draws, while Beth’s score is the square of the number on the card she draws. Find the value of $k$ so that the game is a fair one.

Apr182017 by KS TengNo Comments

Differentiation Question #1

JC Mathematics

Given that $y = \frac{8}{x^3} - \frac{6}{x^2} + \frac{5}{2x}$ , find the approximate percentage change in $y$ when $x$ increases from 2 by 2%.

Hint

Apr52017 by KS TengNo Comments

Probability Question #4

JC Mathematics

A gambler bets on one of the integers from 1 to 6. Three fair dice are then rolled. If the gambler’s number appears $k$ times ( $k = 1, 2, 3$ ), he wins $ $k$ . If his number fails to appear, he loses $1. Calculate the gambler’s expected winnings

Hint

Mar302017 by KS TengNo Comments

Vectors Question #4

JC Mathematics

Another interesting vectors question.

The fixed point $A$ has position vector a relative to a fixed point $O$ . A variable point $P$ has position vector r relative to $O$ . Find the locus of $P$ if r $\bullet$ (r – a) = 0.

Solution

Mar242017 by KS TengNo Comments

Vectors Question #3

JC Mathematics

This is a question a student sent me a few days back, and I shared with my class.

Find the Cartesian equation of the locus of all points (plane) that is equidistant of the $xy$ plane and $xz$ plane.

The following should aid students to visualise.

Hint

Solution

Alternative

Sidenote: I think Vectors is a very important topic for 9758 as its applications are wide. Students should do their best to understand the topic. I will share a few more applied questions next week when I have time.

k	1	2	3
$P(X=k)$	$^3C_1(\frac{1}{6})(\frac{5}{6})^2$	$^3C_2(\frac{1}{6})^2(\frac{5}{6})$	$^3C_3(\frac{1}{6})^3$