1. log a (xy) = loga x + loga y
3. logx x = 1
4. loga 1 = 0
5. loga (xn) = n(loga x)
1. Characteristic
The internal part of the logarithm of a number is called its characteristic.
Case I: When the number is greater than 1.
Case II: When the number is less than 1.
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).
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".
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
Replacing x by −x in the logarithmic series, we get
(1)
(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,
(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.
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