Logarithm Model Questions Set 1 Practice Questions Answers Test with Solutions & More Shortcuts
Logarithm PRACTICE TEST [1 - EXERCISES]
Logarithm Model Questions Set 1
Question : 16
What is the value of $(log_{1/2}2)(log_{1/3}3)(log_{1/4}4).......(log_{1/{1000}}1000)$ ?
a) 1 or –1
b) 1
c) –1
d) 0
Answer »Answer: (c)
$(log_{1/2}2)(log_{1/3}3)(log_{1/4}4).......(log_{1/{1000}}1000)$
= $({log 2}/{log_{1/2}})({log 3}/{log_{1/3}})({log 4}/{log_{1/4}})......({log 1000}/{log_(1/{1000})})$
$(∵ log_b a = {log a}/{log b})$
= $({log 2}/{- log 2})({log 3}/{- log 3})({log 4}/{- log 4}).........({log 1000}/{- log 1000})$
= (-1) × (-1) × (-1) × ..... × (-1)
(∵ number of factors is odd)
= –1
Question : 17
If ${\text"log"_a}$ (ab ) = , then $\text"log"_b$ (ab) is
a) $x/{x - 1}$
b) $1/x$
c) $x/{1 - x}$
d) $x/{x + 1}$
e) None of these
Answer »Answer: (a)
${\text"log"_a}$(ab) = x
⇒${\text"log" ab}/{\text"log" a} = x⇒{\text"log" b}/{\text"log" a} = x - 1⇒{\text"log" a}/{\text"log" b} = 1/{x - 1}$
Now, ${\text"log"_b}(ab) = {\text"log" ab}/{\text"log" b} = {\text"log" a + \text"log" b}/{\text"log" b}$
= ${\text"log" a}/{\text"log" b} + 1 = {1/{x - 1}} + 1 = {1 + x - 1}/{x - 1} = {x}/{x - 1}$
Question : 18
If $log_r 6 = m and log_r 3$ = n, then what is $log_r (r/2)$ equal to?
a) 1 – m – n
b) m – n + 1
c) m + n – 1
d) 1 – m + n
Answer »Answer: (d)
Given, $log_r 6 = \text"m and" log_r$ 3 = n
∵ $log_r 6 = log_r$ (2 × 3)
= $log_r 2 + log_r$ 3
∴ $log_r 3 + log_r$ 2 = m
⇒n + $log_r2$ = m
⇒$log_r$ 2 = m – n
∴ $log_r (r/2) = log_r r - log_r 2$
= 1 – m + n
Question : 19
Let XYZ be an equilateral triangle in which XY = 7 cm. If A denotes the area of the triangle, then what is the value of $log_{10}A^4$ ? (Given that $log_{10}1050 = 3.0212$ and $log_{10}35$ = 1.5441)
a) 5. 5635
b) 5. 3070
c) 5. 3700
d) 5. 6535
Answer »Answer: (b)
$log_{10} 1050 = log_{10}$(3 × 10 × 35)
= $log_{10} 3 + 1 + log_{10} 35$
⇒ 3.0212 = $log_{10} 3 + 1 + 1.5441$
⇒$log_{10} 3$ = 0.4771
Now, $log_{10} 35 = log {7 × 10}/2$
= $log_{10} 7 + log_{10} - log_{10} 2$
⇒ $log_{10}7$ = 0.8451
Now, A = ${√3}/4 × (7)^2$
$log{10^A^4} = 4 log_{10}A$
= 4 log ${√3 × 7^2}/4$
= $4[1/2 log_3 + 2 log_{10} 7 - 2 log 2]$ = 5.3070
Question : 20
What is the logarithm of 0.0001 with respect to base 10?
a) –4
b) 4
c) 3
d) –3
Answer »Answer: (a)
Let $log_{10}$ 0.0001 = a
a = $log_{10} {1/{(10)^4}$
= $log_{10} 1 – log_{10} (10)^4$ = 0 – 4 = –4
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