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Important Formulas: Arithmetic Progression | Quantitative for GMAT PDF Download

Logarithm

  • If a is a positive real number, other than 1 and am = x, then we write: m = loga x and we say that the value of log x to the base a is m.
  • Examples: (i) 103 1000 ⇒ log10 1000 = 3.
    (ii) 34 = 81 ⇒ log3 81 = 4.
    Important Formulas: Arithmetic Progression | Quantitative for GMAT
    (iv) (. 1)2 = 01 ⇒ log(.1) .01 = 2. 

Properties of Logarithms

1. log a (xy) = loga x + loga y

Important Formulas: Arithmetic Progression | Quantitative for GMAT

3. logx x = 1 

4. loga 1 = 0 

5. loga (xn) = n(loga x)

Important Formulas: Arithmetic Progression | Quantitative for GMAT

Common Logarithms

  • Logarithms to the base 10 are known as common logarithms.
  • The logarithm of a number contains two parts, namely 'characteristic' and 'mantissa'.

1. Characteristic

The internal part of the logarithm of a number is called its characteristic.
Case I: When the number is greater than 1.

  • In this case, the characteristic is one less than the number of digits in the left of the decimal point in the given number.

Case II: When the number is less than 1.

  • In this case, the characteristic is one more than the number of zeros between the decimal point and the first significant digit of the number and it is negative.
  • Instead of -1, -2 etc. we write Important Formulas: Arithmetic Progression | Quantitative for GMAT (two bar), etc.
    Examples:
    Important Formulas: Arithmetic Progression | Quantitative for GMAT

2. Mantissa:

The decimal part of the logarithm of a number is known is its mantissa. For mantissa, we look through log table.

Question 1: If log2X + log4X = log0.25√6 and x > 0, then x is:

A. 6-1/6
B. 61/6
C. 3-1/3
D. 61/3

Correct Answer is Option (A).

  • log2x + log4x = log0.25√6
    We can rewrite the equation as:  
    Important Formulas: Arithmetic Progression | Quantitative for GMAT
     log2x * 3 = 2log0.25√6
     log2x3 = -log46 
    Important Formulas: Arithmetic Progression | Quantitative for GMAT
    Important Formulas: Arithmetic Progression | Quantitative for GMAT
     2log2x3 = -log26
  • 2log2x3 + log26 = 0
    log26X6 = 0
  • 6x6 = 1 
    x6 = 1/6
    Important Formulas: Arithmetic Progression | Quantitative for GMAT
  • The question is "If log2X + log4X = log0.25 √6 and x > 0, then x is"
  • Hence, the answer is "6-1/6".

Question 2: log9 (3log2 (1 + log3 (1 + 2log2x))) = 1/2. Find x. 

A. 4
B. 1/2
C. 1
D. 2
Correct Answer is Option (D).
log9 (3log2 (1 + log3 (1 + 2log2x)) = 1/2

3log2(1 + log3(1 + 2log2x)) = 91/2 = 3
log2(1 + log3(1 + 2log2x) = 1 
1 + log3(1 + 2log2x) = 2 
log3(1 + 2log2x) = 1
1 + 2log2x = 3 
2log2x = 2 
log2x = 1 
x = 2 
The question is "Find x."
Hence, the answer is "2".


Question 3: If 22x+4 – 17 × 2x+1 = –4, then which of the following is true? 
A. x is a positive value
B. x is a negative value
C. x can be either a positive value or a negative value 
D. None of these 
Correct Answer is Option (C).

2x+4 – 17 * 2x+1 = – 4 
=> 2x+1 = y 
22x+2 = y2
22(22x+2) – 17 * 2x+1 = –4 
4y2 – 17y + 4 = 0 
4y2 – 16y – y + y = 0 
4y (y – 4) – 1 (y – 4) = 0 

y = 1/4 or 4

2x+1 = 1/4 or 4

⇒ x + 1 = 2 or – 2
x = 1 or – 3

The question is "which of the following is true?"
Hence, the answer is "x can be either a positive value or a negative value".

Logarithmic Series

Definition

An expansion for loge (1 + x) as a series of powers of x which is valid only, when |x|<1.

Expansion of logarithmic series

Expansion of loge (1 + x) if |x|<1 then
Important Formulas: Arithmetic Progression | Quantitative for GMAT

Replacing x by −x in the logarithmic series, we get
Important Formulas: Arithmetic Progression | Quantitative for GMAT

Some Important results from logarithmic series

(1) 
Important Formulas: Arithmetic Progression | Quantitative for GMAT
Important Formulas: Arithmetic Progression | Quantitative for GMAT

(2) The series expansion of loge (1 + x) may fail to be valid, if |x| is not less than 1. It can be proved that the logarithmic series is valid for x = 1. Putting x = 1 in the logarithmic series.

We get,
Important Formulas: Arithmetic Progression | Quantitative for GMAT
(3) When x = −1, the logarithmic series does not have a sum. This is in conformity with the fact that log(1 – 1) is not a finite quantity.

Difference between the exponential and logarithmic series

  1. In the exponential series Important Formulas: Arithmetic Progression | Quantitative for GMAT all the terms carry positive signs whereas in the logarithmic series Important Formulas: Arithmetic Progression | Quantitative for GMAT the terms are alternatively positive and negative
  2. In the exponential series the denominator of the terms involve factorial of natural numbers. But in the logarithmic series the terms do not contain factorials.
  3. The exponential series is valid for all the values of x. The logarithmic series is valid, when |x|< 1.
The document Important Formulas: Arithmetic Progression | Quantitative for GMAT is a part of the GMAT Course Quantitative for GMAT.
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FAQs on Important Formulas: Arithmetic Progression - Quantitative for GMAT

1. What are the properties of logarithms?
Ans. The properties of logarithms are as follows: - Product Rule: logₐ(xy) = logₐ(x) + logₐ(y) - Quotient Rule: logₐ(x/y) = logₐ(x) - logₐ(y) - Power Rule: logₐ(xⁿ) = n * logₐ(x) - Change of Base Rule: logₐ(x) = logᵦ(x) / logᵦ(a) - Inverse Rule: If logₐ(x) = y, then aⁿ = x
2. What are the important formulas related to arithmetic progression?
Ans. The important formulas related to arithmetic progression (AP) are: - nth term of an AP: aₙ = a + (n-1)d - Sum of first n terms of an AP: Sₙ = (n/2)(2a + (n-1)d) - Sum of first n terms of an AP using the last term: Sₙ = (n/2)(a + l) - Sum of squares of first n natural numbers: Sₙ = (n/6)(n+1)(2n+1)
3. How do you use the product rule of logarithms?
Ans. To use the product rule of logarithms, you can split a product inside a logarithm into separate logarithms. For example, if you have logₐ(xy), you can rewrite it as logₐ(x) + logₐ(y). This allows you to simplify the calculation by evaluating the logarithms of the individual factors separately and then adding them.
4. How do you find the nth term of an arithmetic progression?
Ans. To find the nth term (aₙ) of an arithmetic progression (AP), you can use the formula aₙ = a + (n-1)d, where a is the first term and d is the common difference between consecutive terms. By substituting the values of a and d into the formula, you can calculate the desired term of the AP.
5. How do you find the sum of the first n terms of an arithmetic progression?
Ans. To find the sum (Sₙ) of the first n terms of an arithmetic progression (AP), you can use the formula Sₙ = (n/2)(2a + (n-1)d), where a is the first term, d is the common difference, and n is the number of terms. By plugging in the values of a, d, and n into the formula, you can calculate the sum of the desired terms.
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