# Exam-PCM (Class XI and XII)

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## Tricks to Guess Correct Answers for MCQs in JEE Main & JEE Advanced

1) Gamble on One of the Two Options

If you have already crossed out two incorrect answers, gamble on one of  the two options in which you are confused. Use other tips to choose your pick as given below.

2) No Negative Marking means You are in the Safe Zone

In case, there are questions like ‘Match the Following’ o other similar questions which do not carry negative marking, you are free to play. Never leave such questions unattempted. Study the marking scheme closely and also read the instructions given in the question paper carefully.

Questions related to finding domain of a function in IIT JEE are usually asked as below:

If x = {0, 1, 2, 3, 4}, y = {2, 3, 4, 5} and x  X, y    Y, then which of the following is a function in the given set -

(a) f1 = {(x,y) : y = x + 1}

(b) f2 = {(x,y) : x + y = >5}

(c) f3 = {(x,y) : y ≤ x}

(d) None of these

In such questions, you do not have to solve the entire function.

Just put in the extreme values of the set in the function and see if
they satisfy the answer. In this question, you can put 0 and 4 for x and 2 and 5 for y. You will be able to get the correct answer – (d) in this case.

I wonder whether administrators put such questions only to mock students who actually solve them. Sometimes the options given as answers are just so laughable! Just the dimensional analysis of the weirdest questions will lead you to the correct answer – quickly and accurately.

Let’s say, there is a question in which you have to find velocity of an electromagnetic field with weird orientation. Its field values are given as B and E. The options are given as:

(a) (2E/B) 1/2

(b) (2B/E)

(c)  (2E/B)

(d) (2B/E) 1/2

• Dimensions of an Electric Field are (M1L1T-3A-1), and
• Dimensions of a Magnetic Field are (M1L0T-2A-1).

Hence, only E/B gives you the Dimension of Velocity, which is (L1T-1).  We do not need the square root as it will lead to the root of velocity too.

So, our answer is (c) in this case. You do not really need to check for ‘2E’ as only one option gives you correct dimensions.
5) Putting Values to Narrow Down your Options

Use this trick for questions in which you have to find general expressions for certain scenarios.

Let’s take an easy example. You have to find the expression for Pressure on a Bubble of Radius R in a container situated at H height from the ground level. The water in the container is up to height L.

(a)    P + [pg (L-h)/LR]
(b)   P + [pg LR/(L-h)]
(c)    P + [pg R (L-h)/L]
(d)   P + [pg (L-h)(L-R)/L)

The dimensional analysis will tell you that the first option is not
correct. Similarly, the second option too gives you a dimension of
Pressure per Unit Area and not just Pressure.

Now, we only have third and fourth options in hand.

We know that the Pressure on the surface of water must be P and (P + pg L) at the base of the container.

At the surface of water, h = L and Pressure = P. At the base of the
container, h = 0 and Pressure = (P + pg L). Put these two values in the equations and you will find the correct answer – (d) in this case.
6) Finding out n-terms in Mathematics

Perhaps the easiest questions in Maths, these either ask for the nth term or the sum or product of n-terms. The value of ‘n’ is mostly 1, 2 or at the most 3. Simply, put in these values and check whether they fit in.

If you have tried up to ‘3’ and you still do not have the answer, leave it alone. It is most likely to be a high-level question.

Here’s an example for you:

The sum of n natural numbers (1+2+3+….+n) is:

(a) n (n-1)/2
(b) n (n+1)/2
(c) n (n+2)/3
(d) (n+1) (n-1)

Sum of 1 as a natural number should be 1 too.

Put 1 as the value of n. Solving the equations you will get 0 for (a), 1 for (b), 1 for (c) and 0 for (d). Hence, we can rule out (a) and (d).

Now we put 2 as the value of n. Solving (b) equation, we get 3 as the answer while (c) equation dissolves into 8/3, which is not a natural number. Hence, the correct answer is (b).
7) Generalize Questions

These are the favorite questions of JEE administrators. They have
complex orientation and you will have to find one of the variables.

Just assume that two or more of the arbitrary values are zero, one or some other easy value and check your options again.

Here’s an example:

For positive real values, it is given that m3x4 = n3y4 and m2 < n2, then

(a)    m3x2 > n3y2
(b)   m3 < y4
(c)    m3x2 < n3y2
(d)   m3 > y4

Solving these will lead you to the hot soup of inequalities.

Hence, here’s an alternative. Assume some easy values for m, x, and n.
Let’s say, m = 2, n = 3, and x = 3. You will get y as the fourth root of 24. Approximate it as the square root of 5 (since 5 is the square root of 25). Hence, it can be roughly calculated to be 2.2.

For such questions, approximating values is a good idea.

Put these values in the equation. Cancel out ‘3’ wherever possible and you will soon find the correct answer as (b).

8) Try it Out

In questions where you get random inclinations of a plane (alpha, beta or whatever), check what will happen if the inclination is zero/90. Put zero or 90 as the value of the plan and see which of the answers satisfy the condition.

You can apply the same logic to find Moment of Inertia about a random axis. Here’s an example:

Find Moment of Inertia (M.O.I) of a rod along an axis at theta degrees from its length passing through the centre:

(a) ML2sin2Ө/12
(b) ML2cos2Ө/12
(c) ML2sinӨcos2Ө/12
(d)ML2sin2ӨcosӨ/12

We know that the M.O.I of a rod along its length is zero. So if Ө = 0, M.O.I = 0 too. Assume Ө = 0 and (a), (c) and (d) all turn out to be zero.

Also, the M.O.I of a rod perpendicular to its axis (for Ө=90) = ML2/12. Using Ө = 90, only (a) gives you that answer. (c) and (d) turn out to be zero. So, (a) is the correct answer.
9) Easiest Topic that fetches you 20-25 Marks

Modern Physics with its atoms, nuclei, radiation and all cannot be just ignored. It fetches you as much 20-25 marks in IIT entrance level exams and you hardly need more than a day or two to understand it all.

Just cram up all the formulae. You just can’t let these questions go.
Strangely, it requires one-tenth of the preparation time you need for Mechanics but fetches you ten times the marks.

10) Diagrams for Co-ordinate Geometry

Even if you don’t thoroughly understand a concept, drawing an
approximate diagram according to the given scale can help you work out the radius, area, equations etc. easily. It might seem like a trivia but it really works in many cases.

11) Re-read Chapter 1 of Physics from your Class 11 NCERT Book

I wonder why no coaching institute pays much attention to this
introductory chapter on Measurements. It is so easy and you get at least three questions from Dimensions, Approximation and finding Maximum Error sections.

Make sure you understand it clearly. Read the study material, go through the answer keys and check past year papers with solutions. It is easy and fetches you some ‘free’ marks.
12) NCERT for Chemistry

My teachers in Class XI and XII always used to say that NCERT books are the ‘Bible’ for all competitive entrance exams in India. And I, as usual, did not pay attention to what they said.

To my horror, I discovered what they said was so true!

So, here are my words of wisdom: Consider NCERT books as the books from where questions will be lifted from – directly. In Maths and Physics, there might be some questions from other books but in Chemistry – every question asked in IIT JEE was from the NCERT book. In fact, in Inorganic Chemistry, some of the lines were directly quoted from the NCERT book –

straight away Ctrl+C and Ctrl+V factor.

Pull out some sample IIT JEE papers. Use these tricks to derive the
answers to the questions in topics you have not studied yet and check your score!~
That reminds me to give you one more warning and one more tip:

Warning: Never carry a book or revision notes to the exam centre. It will just confuse you and stress you out in the last minute. Just carry your admit card and a pencil box/geometry box with the necessary stationery. And get at least six hours of sleep the night before the exam.

Tip: When you feel your spirits floundering in despair, just plug in your earphones and listen to these songs:

In English:
• Firework – Katy Perry
• Live like you’re dying – Kris Allen
• Lose Yourself- Eminem
• Remember the name – Fort Minor
• Someday – Flipsyde
• Who says – Selena Gomez (meant for girl aspirants)
In Hindi:
• Baar baar haan, bolo yaar haan – Lagaan
• Jo soche jo chahe woh kar ke dikha de – Do aur do Paanch
• Koi Kahe Rehte Rahe – Dil Chahta Hai
• Kya Karoon – Wake Up Sid
• Lakshya (the title song) – Lakshya
• Yeh Honsla Kaise Jhuke – Dor
All the best for your JEE exams! I hope these tips help you in cracking the exam.
This blog content is originally shared from IIT AND KOTA : 15 Tips to Cheat at MCQs in IIT JEE: Guessing Your..