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Need assistance with C3 trigonometry proof question

Posted by Whitecorp on June 21, 2014 at 9:05 PM Comments comments (0)


Please could you help me with answering the following proof question for trigonometry in C3.


23) Prove that if P, Q and R are the angles of a triangle, then


Student X

P+Q+R=180 deg


P+Q = 180 deg -R


tan(P+Q) = tan (180 deg -R)


(tanP +tanQ)/ (1- tanP tanQ) =-tanR


(Note: tan 180 deg =0 )


Proceed with a little more housekeeping of the above, and you should arrive at the required proof shortly.


Hope this helps. Peace.

Best Regards,

Mr Koh

Need help with RSA Cryptography

Posted by Whitecorp on June 11, 2014 at 9:40 AM Comments comments (0)

I' m studying computer science and engineering , but I wasn't able to follow the Math lectures, and now I`m having problems understanding some Math tasks.



Within 2 days I have an exam and I need to pass it , please help me in understanding this.



The task is :


Using Rules of RSA Cryptography by having " p = 5 , q= 13 , and  e=7 to DECRYPT this message




Student X

I will get you started off then. Let's decode 59:


p*q = 5*13 =65


φ(n)= (p-1)*(q-1) = 4*12 =48


The modular multiplicative inverse of e (mod φ(n)) where e=7, φ(n)=48 gives the result of 7.


59^7 mod(65) =2488651484819 mod (65) =19


For each of the remaining encrypted messages, the corresponding decrypted message is simply given by the value of M^7 mod (65), where M is the encrypted message.


Hope this helps. Peace.

Best Regards,

Mr Koh

Sixth Term Examination Paper II

Posted by Whitecorp on June 6, 2014 at 7:50 AM Comments comments (0)

Hi White Group Maths,

This is Student X here, a soon to be student of mathematics in university.

In about a month, I'm going to take the STEP II paper. In preperation for this, I've been doing practice questions since February, roughly 2 hours a day.

However, despite this practice... I feel no more confident than I did 3 months ago, and am quite worried. Do you have any advice on how to further prepare for this test? Or any tuition services to to suggest?

Thanks so much for the help.


Student X

Hi there ,


                 Thanks for writing. It is encouraging to know you have commenced preparations for the STEP paper way before the actual sitting itself; it is perfectly normal to feel shaky despite months of drilling, because STEP isn't your conventional A Level exam to begin with-it demands the candidate to constantly expect the unexpected, and to be able to conjure solutions bordering on ingenuity. Above all, the compulsory possession of a rock solid Mathematical foundation so as to nimbly navigate various kinds of twists and turns.


                 Your exposure to various genres of problems by virtue of the efforts invested since February should be considerable by now, however you do need to ask yourself these three questions:


1. What percentage of the paper did you manage to complete properly in recent attempts? Can things be improved? (Time management)


2. Do you tend to linger around a problem for way too long before moving on? Have you learnt to not allow frustration overwhelm the big picture? (Cutting losses)


3. Did you make it a point to acquire the underlying moral of the story for particularly intense problems? ( enhancing solving efficiency and becoming more elegant in solution design)



Just to add, unlike the A Level papers, you may not have the luxury of excess time at the end to verify the correctness of previous scripted answers, so you must tread carefully from the onset.


                       I doubt tuition is what you really needat this very moment-it's more about readying your state of mind for the big event. An extremely nervous constitution can wreak substantial damage to your performance on that day itself, so start conditioning yourself to be cool-headed (of course easier said than done, but it has to be done).Stop worrying about the final grade you will receive or the somewhat insurmountable difficulties lying in the immediate future, try to "enjoy" the process of conquering the paper instead.


                        And do remember to have an early rest on the night before the actual STEP examination. Good luck, and god bless.



Best Regards,

Mr Koh


Evaluating an integral without actually solving it

Posted by Whitecorp on May 8, 2014 at 10:00 AM Comments comments (0)

Is there any way to obtain the solution for the below integral without solving it?

Student X

Certainly. The function to be integrated is odd in nature.

If we let f(x)= x* e^|x| *sec x, 

then f(-x) = (-x)* e^|-x| *sec (-x) =  - x* e^|x| *sec x = - f(x)

This implies that the graph of f(x) has rotational symmetry about the origin. (A simple example would be the sine curve) . Thus, if f(x) is integrated wrt x from -a to a for all a ∈ ℝ, then the result is simply zero.

Hope this helps. Peace.

Best Regards,

Mr Koh

Rate of change problem

Posted by Whitecorp on April 2, 2014 at 10:35 AM Comments comments (0)

The circle below has a radius of 6 cm and point C is moving clockwise around the circle.  Assume that point B is moving away from point A at a rate of 2 cm./sec. i.e. x is increasing at a rate of 2 cm./sec.  At this moment , assume θ is pi/3 radians.

a. What is the instantaneous rate of change of θ at this moment. i.e. how fast is θ decreasing at this moment?

b. Also, at this moment, is θ decreasing faster or slower?  By how much?

Student X

At any particular instant, let's focus on the right angled triangle ABC where AB= x units and angle CAB = θ, where both x and θ varies with time.


A relationship can be formed between x and theta, which is


x = 6 cos θ


Differentiating both sides wrt y gives


dx/dt = - 6 sin θ * dθ/dt -------(1)


Substituting dx/dt =2 cm/s, θ = π/3,


instantaneous rate of change of θ at this moment


= dθ/dt = 2 ÷ [ -6 * sqrt(3) /2 ] =-0.385 rad/s (shown)


Since x increases at a constant rate of 2cm/s, its second order derivative wrt time is zero, ie d^2 x/dt^2 = 0 cm/s^2


Differentiating (1) on both sides wrt t once again,



d^2 x/dt^2 =  -6 [ cosθ *( dθ/dt)^2 + sinθ * (d^2 θ/dt^2 )]


(Be mindful of how the product rule is employed in the RHS)



Substituting d^2 x/dt^2 = 0 cm/s^2, θ = π/3  and  dθ/dt = -0.385 rad/s,



0 =  -6 [ cos(π/3) *( -0.385)^2 + sin(π/3)  * (d^2 θ/dt^2 )]


Solving gives d^2 θ/dt^2 = -0.0856 rad/s^2


Hence, at this moment, θ is decreasing slower (because of the negative sign) by 0.0856 rad/s. (shown)



Hope this helps. Peace.

Best Regards,

Mr Koh






Qualitative understanding of variance formula

Posted by Whitecorp on March 11, 2014 at 10:35 AM Comments comments (0)


           I don't quite get why:

Var (U-V)= Var(U) + Var (V) - 2Cov( U, V)

I get the covariance bit but why isn't it Var(U) - Var (V) ?

Student X

Understand that every variable introduced involves a certain amount of uncertainty (some may like to call it errors); the more variables, the greater the uncertainty. In this regard, separate variances of variables will reinforce each other as opposed to eliminating each other. Covariance is characterized as a quantity removed as uncertainty overlaps occur when one variable is somewhat dependent on the other.


Hope this clarifies. Peace.

Best Regards,

Mr Koh

Finishing touches to the solution of a differential equation

Posted by Whitecorp on March 10, 2014 at 9:45 AM Comments comments (0)

I have been following the workings in my textbook on autonomous equations and can follow everything except for one step: solving the solution for y.


The general solution is:


To satisfy the initial condition  

        ,   ,


Despite having been working on this for a good deal of time, I simply can not see how they have solved for y, giving:




Could you tell me how this is reached?

Student X



Inverting both sides concurrently yields

Now, making use of the substitution



we have



Hope this helps. Peace.

Best Regards,

Mr Koh

Ratio word problem

Posted by Whitecorp on March 9, 2014 at 9:50 PM Comments comments (0)



I am currently retaking my Maths GCSE atm on a distance learning course. The course lacks a lot of depth in the way they explain things, and doesn't show how they got the solutions, it just solely states the solution. I am stuck on this ratio word problem, would you be willing to help?


Here's the question:


The hotel bill for 4 people staying 7 days comes to £630. How much would 3 people who stayed 10 days have to pay?


I have developed several different answers but none of them are correct. Any help would be much appreciated.


Thanks for reading.:)

Student X

You can break things down by systematic reasoning. Some food for thought:


If the hotel bill for 4 people staying 7 days comes to £630, then 4 people staying for 1 day would need pay £630/7 =£ 90


In consideration of this, what happens if there is only a single person staying at the hotel for 1 day?


Naturally he only pays his share which is equivalent to £ 90/4 =£ 22.5


On the other hand, should this loner choose to stay at the hotel for a stretch of 10 days, the cost would come to £ 22.5*10 =£ 225


I will leave you to figure out the rest, which shouldn't be all that difficult at this point.


Hope my explanation helps. Peace.

Best Regards,

Mr Koh

Linear Programming Problem

Posted by Whitecorp on February 28, 2014 at 10:05 AM Comments comments (0)

A dog food manufacturer makes 3 types of dog chew, each 10g in weight, which are made from different proportions of 2 basic ingredients. The table below shows this, together with the amount of the two ingredients in stock, and the costs for the three types of chew.


chew type | ingredient 1 | ingredient 2 | cost (p per chew)|

type A                   8g                 2g                      1.8

type B                   6g                 4g                      1.6

type C                   5g                 5g                      1.5

availability            800kg             400kg


The manufacturer wants to make 1600 packets of mixed chews. Each must contain 60 chews, and there must be no more than 30 of each type of chew in a packet.


a) Define appropraite variables and formulate an LP showing that:




b) By eliminating the variable z show that the objective function can be given by  c = 90+0.3x+0.1y

and hence define the problem in terms of 2 variables.


c) By using a graphical approach solve the LP stating all the possible solutions.


d) From your graph you should see that 2 of the calculated constraints are unnecessary. Which constraints are these?



Student X

Here's getting you started:


Let x, y and z represent the number of each type of chews A, B and C respectively in a packet.


Since 1600 packets are to be produced and a maximum of 800kg of ingredient 1 can be used in total, 800/1600= 0.5 kg or 500 g is the maximum amount of ingredient 1 which can be used in a single packet.



Similarly, since 1600 packets are to be produced and a maximum of 400kg of ingredient 2 can be used in total, 400/1600= 0.25 kg or 250 g is the maximum amount of ingredient 2 which can be used in a single packet.


Hence, since the total amount of ingredients 1 and 2 used in making a single packet of chews are separately represented by 8x+6y+5z and 2x+4y+5z respectively, imposing the above conditions would yield



2x+4y+5z<=250 (shown)


We also know that since each packet must contact exactly 60 chews,

then  x+y+z=60 ====> z = 60 -x -y---------(1)


The cost involved in manufacturing one packet is c = 1.8x + 1.6y + 1.5z --------------(2)


Substituting (1) into (2):


c = 1.8x + 1.6y + 1.5(60 -x -y) = 1.8x +1.6y + 90 -1.5x -1.5y


   = 90 +0.3x +0.1y (shown)



Hopefully I have given you sufficient reference to continue working on your own through the remaining parts of the question which should be pretty manageable. Peace.


Best Regards,

Mr Koh

Need help for this JC 1 polynomial question

Posted by Whitecorp on February 15, 2014 at 9:50 PM Comments comments (0)



Please help me in this math question-!!

The polynomial f(x) has a factor (x - 1) and leaves a remainder of 3 when divided by (x + 2).

Given that the following relation f(x +1) + f(x +4) = a is true for all real values of x, find the value of a.

I didn't understand the back part of the question and I am lost now so please help me.

Student X

f(1)=0 since x-1 is a factor and  f(-2)= 3 since f(x) yields a remainder of 3 when divided by x+2.

f(x +1) + f(x +4) = a is true for all values of x implies we can choose any value of x, substitute into the LHS expression and the RHS will always evaluate to the value of a.

So choosing x=-3,

f(-3+1) + f(-3+4) = f(-2) + f(1) = 3 + 0 = 3

Therefore, the value of a is 3. (shown)

Hope this helps. Peace.

Best Regards,

Mr Koh