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Quant – Tips, Tricks, Shortcuts & Formulae

Quant Short-tricks & Formulae

Multiplication

Multiplying 2-digit numbers

Type-I

AB × CD = A × C / A × D + B × C / B × D

E.g. 1.   34 × 57 = 3 × 5 / 3 × 7 + 4 × 5 / 4 × 7
= 15 / 21 + 20 / 28
= 15 / 41 / 28
= 1(5 + 4)(1 + 2)8
= 1938

E.g. 2.   56 × 48 = 5 × 4 / 5 × 8 + 6 × 4 / 6 × 8
= 20 / 40 + 24 / 48
= 20 / 64 / 48
= 2(0 + 6)(4 + 4)8
= 2688

Type-II

AB × AC = A2 / A (B + C) / B × C

E.g. 1.   64 × 65 = 62 / 6(4 + 5) / 4 × 5
= 36 / 54 / 20
= 3(6 + 5)(4 + 2)0
= 4180

E.g. 2.   54 × 56 = 52 / 5(4 + 6) / 4 × 6
= 25 / 50 / 24
= 2(5 + 5)(0 + 2)4
= 3024

Type-III

AB × CC = AC / (A + B)C / BC

E.g. 1.   24 × 33 = 2 × 3 / (2 + 4) × 3 / 4 × 3
= 6 / 18 / 12
= (6 + 1)(8 + 1)2
= 792

E.g. 2.   43 × 55 = 4 × 5 / (4 + 3) × 5 / 3 × 5
= 20 / 35 / 15
= 2(0 + 3)(5 + 1)5
= 2365

Multiplying 3-digit numbers

ABC×DEF=A×D/A×E+B×D/A×F+B×E+C×D/B×F+C×E/C×F

E.g.321×546=3×5/3×4+2×5/3×6+2×4+1×5/2×6+1×4/1×6
= 15 / 12 + 10 / 18 + 8 + 5 / 12 + 4 /0 6
=  15 / 22 / 31 / 16 / 06
= 1(5 + 2)(2 + 3)(1 + 1)(6 + 0)6
= 175266

Easy techniques for multiplication

To multiply with 4, multiply with 2 twice
To multiply with 5, multiply with 10 and then divide by 2
To multiply with 6, multiply with 3 and then 2
To multiply with 9, multiply with 3 twice

 

Divisibility test

A number is divisible by 2, if the last digit is even i.e. 0,2,4,6 or 8

A number is divisible by 3, if the sum of all the digits is divisible by 3

A number is divisible by 4, if the last two digits are divisible by 4

A number is divisible by 5, if the last digit is 0 or 5

A number is divisible by 6, if the number is divisible by 2 and 3

A number is divisible by 7, if the new number obtained after removing the last digit and subtracting its twice from the truncated original number, is divisible by 7

A number is divisible by 8, if the last three digits are divisible by 8

A number is divisible by 9, if the sum of all the digits is divisible by 9

A number is divisible by 10, if the last digit is 0

A number is divisible by 11, if the difference of sum of digits at odd and even places is divisible by 11

 

Square of a number

  • No square number ends in 2, 3, 7 or 8.
  • The number of zeros at the end of a perfect square is always even.
  • Square of even numbers are even and squares of odd numbers are odd.
  • A perfect square leaves remainder 0 or 1 on division by 3.
  • A perfect square leaves remainder 0 or 1 on division by 4.
  • The unit’s digit of the square depends on unit’s digit of the number.(See Table)

 

 

Unit’s digit of square of numbers

 

Unit’s digit ofthe number Unit’s digit of the square of number
0 0
1 1
2 4
3 9
4 6
5 5
6 6
7 9
8 4
9 1

 

 

Squares and cubes of first 30 natural numbers

Number Square Cube
1 1 1
2 4 8
3 9 27
4 16 64
5 25 125
6 36 216
7 49 343
8 64 512
9 81 729
10 100 1000
11 121 1331
12 144 1728
13 169 2197
14 196 2744
15 225 3375
16 256 4096
17 289 4913
18 324 5832
19 361 6859
20 400 8000
21 441 9261
22 484 10648
23 529 12167
24 576 13824
25 625 15625
26 676 17576
27 729 19683
28 784 21952
29 841 24389
30 900 27000

 

Square of a number if all the digits are same(1,3 or 9)

112 = 121
1112 = 12321
11112 = 1234321
111112 = 123454321
1111112 = 12345654321
11111112 = 1234567654321
111111112 = 123456787654321

332 = 1089
3332 = 110889
33332 = 11108889
333332 = 1111088889
3333332 = 111110888889
33333332 = 11111108888889
33333332 = 1111111088888889

992 = 9801
9992 = 998001
99992 = 99980001
999992 = 9999800001
9999992 = 999998000001
99999992 = 99999980000001
999999992 = 9999999800000001

 

 

Square root of a number

Complete square root of a number is only possible if its last digit is 0, 1, 4, 5, 6 or 9. If the last digit is 2, 3, 7 or 8 then there is  no complete square root.

Last digit ofthe number Last digit of completesq root
0 0
1 1 or 9
2 No Complete square root
3 No Complete square root
4 2 or 8
5 5
6 4 or 6
7 No Complete square root
8 No Complete square root
9 3 or 7

 

Square roots and cube roots of first ten natural numbers

Number
x
Square Root
x1/2
Cube Root
x1/3
1 1.000 1.000
2 1.414 1.260
3 1.732 1.442
4 2.000 1.587
5 2.236 1.710
6 2.449 1.817
7 2.646 1.913
8 2.828 2.000
9 3.000 2.080
10 3.162 2.154

 

Even and odd numbers

Even + Even = Even
Odd + Odd = Even
Even + Odd = Odd
Even × Even = Even
Odd × Odd = Odd
Even × Odd = Even

Prime numbers

To check if a number is prime

  • Find approximate square root of the number
  • Check if the no. is divisible by any prime number less then its square root
  • If not divisible then the number is a prime number

E.g.  Number 461 is a prime number or not.

Approx sq. root = between 21 and 22

Prime numbers less then square root are 2, 3, 5, 7, 11, 13, 17 and 19

Since 461 is not divisible by any of these, it is a prime number

NOTE:

  1. If complete square root exist then no. is not prime
  2. There are fifteen prime numbers from 1 to 50, twenty five from 1 to 100.

2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41,43, 47

53, 59, 61, 67, 71, 73, 79, 83, 89, 97

Number of divisors

If N is any no. and N = an × bm × cp × …. where a, b, c are prime no.

No. of divisors of N = (n + 1) (m + 1) (p + 1) ….

e.g. Find the no. of divisors of 720

N = 720 = 24 × 32 × 51

No. of divisors = (4 + 1) (2 + 1) (1 + 1) = 30

Percentage

Percentage means per every hundred.

Properties

  • p % of q = q % of p= (p × q)/100
  • What percentage of p is q: q/p × 100
  • Percentage change = (change/initial value) × 100
  • Increase p by q% = p( 1+ q/100 )
  • Decrease p by q% = p(1 – q/100)
  • If the value changes by x%, the percentage change required to bring back to original value is 100x/(100 + x)%.
  • If p is x% more/less than q, then q is 100x/(100 + x)% less/more than p.
  • If there are successive percentage changes of a% and b%, then the effective percentage change is:
    (a + b + (ab/100))%

 

 

Geometry

Triangles and quadrilaterals

Equilateral triangle

A triangle is called equilateral if all the angles or sides are equal

Properties

  • All sides are equal
  • All angles are 60o

Isosceles triangle

A triangle is called isosceles if any two angles or sides are equal

Properties

  • Two sides are equal
  • Two angles are equal

Parallelogram

A quadrilateral is called a parallelogram if opposite sides are parallel, equal or opposite angles are equal.

Properties

  • Opposite sides are parallel and equal
  • Opposite angles are equal
  • Consecutive angles are supplementary
  • A diagonal of a parallelogram divides it into two triangles of equal area
  • Parallelograms on the same base and between the same parallels are equal in area
  • Diagonals bisect each other

Rhombus

A quadrilateral is called a rhombus if all the sides are equal.

Properties

  • All sides are equal and opposite sides are parallel
  • Diagonals bisect the angles at vertex.
  • The diagonals are perpendicular bisectors of each other

Rectangle

A quadrilateral is called a rectangle if all the angles are right angle.

Properties

  • Opposite sides are equal
  • All angles are right angles
  • Diagonals are equal and bisect each other

Square

A quadrilateral is called a square if all the sides are equal and all the angles are right angle.

Properties

  • All sides are equal
  • All angles are right angles
  • Opposite sides are parallel
  • Diagonals are perpendicular bisectors

Note:

Parallelogram –> If all angles are equal –> Rectangle

Parallelogram –> If all sides are equal –> Rhombus

Parallelogram –> If  both angles and sides are equal –> Square

 

Properties of triangle

  • There are six exterior angles in a triangle.
  • Sum of interior angle & corresponding exterior angle is always 180°.
  • Sum of interior opposite angles is equal to exterior angle
  • Sum of the length of any two sides is always greater than the length of the third side.
  • Difference of the length of any two sides is always less than the third side.
  • Side opposite to the greatest angle is the longest side.
  • A triangle must have at least two acute angles.
  • Triangles with same or equal base and between the same parallels have equal areas.
  • If a, b, c denotes the length the sides of a triangle then
    If c2 < a2 + b2, Triangle is acute angled.
    If c2 = a2 + b2, Triangle is right angled.
    If c2 > a2 + b2, Triangle is obtuse angled.
  • Any median divides the triangle in two parts with equal areas.
  • The point of intersection of the medians is called the Centroid of the triangle.
  • The centroid divides any median in the ratio 2 : 1.
  • Midpoint Theorem: In any triangle, the line joining the mid points of any two sides is parallel to the third side and is half in length.

 

 

Properties of polygon

  • Sum of all the exterior angles of a polygon is 360°
  • Each exterior angle of a regular polygon is 360° / n
  • Sum of all the interior angles of a polygon is (n – 2) x 180°
  • Each interior angle of a regular polygon is (n – 2) / n x 180°
  • Number of diagonals of a polygon is n(n – 3) / 2
  • The ratio of sides a polygon to the diagonals of a polygon is 2 : (n – 3)
  • Ratio of interior angle to exterior angle of a regular polygon is (n – 2) : 2

 

 

Properties of circle

  • One and only one circle passes through three fixed points.
  • From a point on the circle only one tangent can be drawn.
  • From any exterior point of the circle only two tangents can be drawn.
  • The lengths of two tangents drawn from an exterior point of the circle are equal.
  • The tangent at any point of a circle and the radius through the point are always perpendicular to each other.
  • If two circles touch each other internally or externally, then their centers & the points of contact are collinear.
  • If two circles touch externally,then the distance between their centers is equal to the sum of the radii of the circles.
  • If two circles touch internally, then the distance between their centers is equal to the difference of the radii of the circles.
  • Circles having same centre are concentric circles.
  • Points lying on the same circle are called concyclic points.

 

Surface area and volume

Lateral surface area and curved surface area are nearly same. It’s called lateral surface area for cube and cuboid and curved surface area for cylinder, cone and hemisphere.

Cube

  • Lateral Surface Area = 4a 2
  • Total Surface Area = 6a 2
  • Volume = a 3

Cuboid

  • Lateral Surface Area = 2(lh + bh) = 2 (l + b) h
  • Total Surface Area = 2(lb + bh + lh)
  • Volume = l × b × h

Cylinder

  • Curved Surface Area = 2πrh
  • Total Surface Area = 2πrh + 2πr  = 2πr(r + h)
  • Volume = πr h

Cone

  • Curved Surface Area = πrl
  • Total Surface Area = πrl + πr  = πr(r + l)
  • Volume = 1/3 πr h

Hemisphere

  • Curved Surface Area = 2πr 2
  • Total Surface Area = 2πr + πr  = 3πr 2
  • Volume = 2/3 πr 3

Sphere

  • Surface Area = 4πr 2
  • Volume = 4/3 πr 3

Frustum

  • Curved Surface Area = π(r +)l
  • Total Surface Area =π(r +)l + πr 2  + πr 2
  • Volume = 1/3 πh (r ++)

 

Algebra

Quadratic equation

The general form of a quadratic equation in x is
ax2 + bx + c = 0 , where a , b , c ∈ R & a 0.

The solution of the quadratic equation , ax² + bx + c = 0 is
x = [– b ± √(b2 – 4ac)]/2a

Discriminant of a quadratic equation

ax² + bx + c = 0 where a, b, c ∈ R & a 0

D = b2 – 4ac

  • If D > 0 ⇔ Roots are real & distinct
  • If D = 0 ⇔ Roots are real & coincident
  • If D < 0 ⇔ Roots are imaginary

 

Important points

  • If all the coefficients of a quadratic equation are real (a, b, c ∈ R & a 0) and p + iq (p , q ∈ R & i = √-1) is one root of the equation, then the other root must be p – i q(conjugate of p + iq) & vice
  • If all the coefficients of a quadratic equation are real (a, b, c ∈ Q & a 0) and p + √q (p , q ∈ Q) is one root of the equation, then the other root must be p – √q & vice
  • A quadratic equation whose roots are α & β is
    (x – α)(x – β) = 0
    x² – (α + β) x + α β = 0
  • A quadratic equation whose sum and product of roots is given can be written as
    x² – (sum of roots) x + product of roots = 0
    x² – (α + β) x + α β = 0
  • The condition that a quadratic function f (x , y) = ax² + 2hxy + by² + 2gx + 2fy + c may be resolved into two linear factors is
    abc + 2fgh – af² – bg² – ch² = 0

 

Logarithmic inequalities

  • For a > 1, the inequalities 0 < x < y & logax < logay are equivalent.
  • For 0 < a < 1, the inequalities 0 < x < y & logax > logay are equivalent.
  • If a > 1, then logax < p ⇒ 0 < x < ap
  • If a > 1, then logax > p ⇒ x > ap
  • If 0 < a < 1, then logax < p ⇒ x > ap
  • If 0 < a < 1, then logax > p ⇒ 0 < x < ap 

 

Algebraic identities

  • (x + y) 2 = x2 + 2xy + y2 =  (x – y) 2 + 4xy
  • (x – y) 2 = x2 – 2xy + y2 =  (x + y) 2 – 4xy
  • x2 + y2 =  (x + y) 2 – 2xy = (x – y) 2 + 2xy
  • x2 – y2 =  (x + y)(x – y)
  • (x + y)3 =  x3 + 3x2y + 3xy2 + y3 =  x3 + y3 + 3xy(x + y)
  • (x – y)3 =  x3 – 3x2y + 3xy2 – y3 =  x3 – y3 – 3xy(x + y)
  • x3 + y3 =  (x + y)(x2 – xy + y2) =  (x + y) 3 – 3xy(x + y)
  • x3 – y3 =  (x – y)(x2 + xy + y2) =  (x – y) 3 + 3xy(x – y)
  • x4 – y4 =  (x – y)(x + y)(x2 + y2)
  • x6 – y6 =  (x + y)(x2 – xy + y2) = (x – y)(x2 + xy + y2)
  • x6 + y6 =  (x2 + y2)(x4 – x2y2 + y4)
  • (x + y + z)2 =  x2 + y2 + z2 + 2xy + 2yz + 2xz
  • (x + y + z)3 =  x3 + y3 + z3 + 3(x + y)(y + z)(z + x)
  • (x – y – z) 2 =  x2 + y2 + z2 – 2xy + 2yz – 2xz
  • x3 + y3 + z3 – 3xyz = (x + y + z)(x2 + y2 + z2 – xy – yz – xz)
  • x4 + x2y2 + y4 =  (x2 – xy + y2)(x2 + xy + y2)
  • (x + a)(x + b) =  x2 + x(a + b) + ab
  • (x – a)(x + b) =  x2 + x(b – a) – ab
  • (x – a)(x – b) =  x2 – x(a + b) + ab

 

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