### Modal value & Expected value

Let us look at the difference between modal value and expected value. We shall start by saying they are different, albeit close.

Modal value refers to the mode, that is, the value that has the highest probability (chance) of occurring.

Expected value refers to the value, we expect to have, on average.

Before we start, I’ll do a fast recap on Binomial Distribution, $X \sim \text{B}(n, p)$ by flashing the formulae that we can find on MF26.

$\text{P}(X = x) = ^n C_x (p)^x (1-p)^{n-x}$

$\mathbb{E}(X) = np$

$\text{Var}(X) = np(1-p)$

The expected value is simply given by $\mathbb{E}(X)$.

Now to find the modal value, we have to go through a slightly nasty and long working. You may click and find out.

We have that $\frac{\text{P}(X = r + 1)}{\text{P}(X = r)} = \frac{(n-r)}{(r+1)} \frac{p}{1-p}$. This is what we call the recurrence formula. We consider this to give us the ratio between successive probabilities. And to illustrate how this works, nothing beats an example question.

Consider candies are packed in packets of 20. On average the proportion of candies that are blue-colored is $p$. It is know that the most common number of blue-colored candies in a packet is 6. Use this information to find exactly the range of values that $p$ can take.

First, most common number is the same as saying the modal/ highest frequency.

This means that $\text{P}(X=6)$ is the highest/ largest probability… Let us turn our attention to the recurrence formula now. If $\text{P}(X=6)$ is the largest, then it means that $\text{P}(X=6) \textgreater \text{P}(X=7)$ and also $\text{P}(X=6) \textgreater \text{P}(X=5)$.

Lets start by looking at the first one… $\text{P}(X=6) \textgreater \text{P}(X=7)$

$\text{P}(X=6) \textgreater \text{P}(X=7)$

$1 > \frac{\text{P}(X=7)}{\text{P}(X=6)}$

$\frac{\text{P}(X=7)}{\text{P}(X=6)} \textless 1$

But hold on! This looks like the recurrence formula. (ok, in exams, its either you use the recurrence formula or derive on the spot. Both works!)

Now I’ll advice you try the second one (before clicking on answer) on your own, that is, $\text{P}(X=6) > \text{P}(X=5)$.

Now, if the question simply says that the expected number of blue-colored candies in a packet of 20 is 6. Then

$\mathbb{E}(X) = 6$

$(20)p = 6$

$p = \frac{3}{10}$

We observe that this value actually falls in the range of $p$ we found.

### DRV questions with a twist

I want to share a question that is really old (older than me, actually). It is from the June 1972 paper. As most students know, Maclaurin’s series was tested in last year A’levels Paper 1 as a sum to infinity. And this DRV did the same thing. Here is the question.

A bag contains 6 black balls and 2 white balls. Balls are drawn at random one at a time from the bag without replacement, and a white ball is drawn for the first time at R th draw.

(i) Tabulate the probability distribution for R.

(ii) Show that E( R ) = 3, and find Var( R ).

If each ball is replaced before another is drawn, show that in this case E( R ) = 4.

### Thoughts on the H2 Mathematics (9758) Papers 2017

Solutions can be found here.

Personal Thoughts: The paper isn’t tedious. Students can do them so long as they know their stuffs. There are several generalising of questions, like question 6 of paper 1. We also saw how conditional probability was actually tested subtly, this tests students’ abilities to reason with guidance (not sure if after this first trial year, will they still guide the students.) Application questions were not tough and well guided. Students can solve it easily if they read it well. Statistics was well crafted and neat.

To be blunt, I’ll give credit to the 9740 H2 Mathematics paper that run concurrently, since it is too tough to set two sets of papers. Its easy to acknowledge that the 9740 (2016) paper was way harder than 9740 (2017). Next year won’t be the same.

Advice: Students should be careful when you revise, make sure you learn, and not do. Understand what you’re doing. The 2017 paper was an inquisitive paper, examiners were watching closely if you pay attention to details, and know your definitions well.

I’ll do an analysis for the paper, you can click on the individual question and read. For students that took the paper, I hope it doesn’t demoralise you.

Paper 1

Paper 2

### 2017 A-level H1 General Paper (8807) Paper 1

Sharing the A-level H1 General Paper (8807) Paper 1…

1. ‘The past is not dead; it is not even past.’ Discuss.
2. Can the use of animals for scientific research ever be justified?
3. In your society, to what extent is it acceptable for public money to be used for the acquisition of works of art?
4. ‘Rehabilitation, not punishment, should be the purpose of the justice system.’ Discuss.
5. Is regulation of the press desirable?
6. Do events, rather than politicians, shape the future?
7. How far is science fiction becoming fact?
8. Examine the role of music in establishing a national identity in your society.
9. To what extent are people judged more by their physical appearance than by their abilities?
10. ‘Practical ability is just as important as intellectual skills.’ How far is this true in your society?
11. Assess the view that attempts to control climate change can never be truly effective.
12. The quality of written language is being destroyed by social media.’ What is your view?
13.

### 2017 A-level H2 Mathematics (9740) Paper 2 Suggested Solutions

All solutions here are SUGGESTED. Mr. Teng will hold no liability for any errors. Comments are entirely personal opinions.

Numerical Answers (click the questions for workings/explanation)

Question 1: $2 \sqrt{15}; xy=6$
Question 2: $d = 1.5;~ r \approx 1.21 \text{~or~} r \approx -1.45;~n=42$
Question 3: $(\frac{1}{2}a, 0), (0,b);~ (a+1, 0,);~ (\frac{a+1}{2}, 0);~ (0, a), (b, 0);~ a = 1;~ gg(x) = x, x \in \mathbb{R}, x \neq 1 , ~ g^{-1}(x) = 1 - \frac{1}{1-x}, x \in \mathbb{R}, x \neq 1;~b= 2 \text{~or~}0$
Question 4: $15.1875;~ \frac{\pi}{2a(a-1)};~ b = \frac{1}{2} + \frac{1}{2}\sqrt{1-a+a^2}$
Question 5: $0.647;~ 0.349;~k=2.56$
Question 6: $955514880;~ 1567641600;~ \frac{1001}{3876}$
Question 7: $31.8075, 0.245;~ p = 0.0139$; Do not reject $h_0$, Not necessary.
Question 8: Model (D); $a \approx 4.18, b \approx 74.0;~ r \approx 0.981$
Question 9: $0.632;~ 1.04 \times 10^{-4};~ 0.472;~ 0.421;~ 0.9408$
Question 10: $0.345;~ 0.612;~ \mu = 12.3, \sigma = 0.475;~ k \approx 55.7$

MF26

### 2017 A-level H2 Mathematics (9740) Paper 1 Suggested Solutions

All solutions here are SUGGESTED. Mr. Teng will hold no liability for any errors. Comments are entirely personal opinions.

Numerical Answers (click the questions for workings/explanation)

Question 1: $ax + (2a - \frac{a^2}{2})x^2 + (\frac{a^3}{3} + 2a - a^2) x^3$; $a = 4$
Question 2: $x \textgreater \frac{1}{\sqrt{b}} + a$ or $x \textless a$
Question 3: 2
Question 4: $a = 4, b =1$; translate the graph 4 units in negative y-direction and translate the graph 2 units in positive x-direction.
Question 5: $a = -1.5, b = 1.5, c = 7$; $x \approx -1.33$; $x \approx -0.145$ or $x \approx 1.15$
Question 6: $r = a + (\frac{d - a \cdot n}{b \cdot n}) b$
Question 7: $(\frac{1}{a}, \frac{1}{ae}); \frac{1}{a^2}$
Question 8: $z = -1 + 2i$ or $z = 2 - i$; $p =-6, q=-66$; $(w^2 - 2w+2)(w^2-4w+29)$
Question 9: $U_n = 2An - A +B$; $A = 3, B =-9$; $k=4$; $\frac{1}{4} (n^4 + 2n^3 + n^2)$ ; $e^x$
Question 10: $a = -4.4$; $R(1.5, 0.5, -1)$; $\frac{1}{2}\sqrt{10}$
Question 11: $\frac{dv}{dt}=c$; $v = 10t +4$; $v = \frac{1}{k}(10- 10 e^{-kt})$

### Relevant materials

MF26

To be honest, this paper is really the same as the H2 Mathematics (9758). They just rephrased everything. You can see for yourself here.

### 2017 A-level H2 Mathematics (9740) Suggested Solutions

Please take note that this is the solutions for H2 Mathematics (9740)

Paper 1

Paper 2

MF26

### Some TYS Questions worth looking at

Prelims Exams was scary. H2 Mathematics isn’t that easy.

Students that had difficulties finishing their prelims exams, should consider working on their time management. The best way to do it, practice 3 hour paper… in a single sitting. And students should note to modify their TYS slightly as several questions in each paper are out of syllabus. In general, we give ourselves 1.5min for every 1 mark.

So here, I’ll share a list of questions that Mr. Wee has compiled. Mr. Wee also wrote e-books recently on solving non-routine problems. They are very interesting and provides the learners a new perspective to solving problems.

Non-routine Problems (Click to link to the solutions)
N2016/P1/Q3
N2016/P1/Q8
N2016/P1/Q10(a)
N2015/P1/Q3
N2015/P1/Q11

Application Questions
N2016/P1/Q9
N2015/P1/Q8
N2014/P1/Q11
Specimen P1/Q9
Specimen P1/Q11
Specimen P2/Q9
Specimen P2/Q10

All the best for your revision!

### RVHS GP Prelims paper 1 2017

1. Is being innovative more desirable than keeping the status quo?
2. ‘The promise of science and technology cannot be realised without the humanities.’ Do you agree?
3. Is politics today nothing but a series of empty promises?
4. ‘Education perpetuates rather than fights inequality.’ Comment.
5. ‘Men only need to be good, but women have to be exceptional.’ To what extent is this true in the workplace today?
6. Is increased military spending justifiable when countries are not at war?
7. Should we always be compassionate?
8. To what extent is renewable energy the solution for the world’s increasing need for energy?
9. Consider the relevance of patriotism in your society today.
10. Given that the global population is growing rapidly, should people be having more children?
11. To what extent are the needs of the marginalised met in your society?
12. ‘There is no such thing as bad art.’ Discuss

### HCI GP Prelims paper 1 2017

1. ‘Nothing but provocation and self-centredness.’ Is this a fair description of the state of affairs in today’s world?
2. ‘My life, my choice.’ How far can people expect to live life this way?
3. Should historical monuments and objects be preserved when such an undertaking is very expensive or even a source of unhappiness?
4. ‘Many receive an education, but few are educated.’ Discuss with reference to situations in your society today.
5. ‘Tourism brings less developed countries more harm than good.’ Comment.
6. How worried should we be that recent advances in science and technology are creating new challenges and worsening old problems?
7. ‘Looks matter, and much more than substance too.’ Would you agree with this claim?
8. ‘The hallmark of a great country is not how prosperous it is, but how inclusive its people can be.’ Should your country work towards this ideal?
9. ‘We must surrender our human rights to win the battle against terrorism.’ Do you agree?
10. ‘Smart cities: innovative, but not necessarily better.’ What do you think?
11. ‘Corporate social responsibility is bad for business and companies should not be expected to take it up. To what extent would you agree?
12. ‘Let us read and let us sin, for what harm can these amusements bring?’ Comment.