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Answers
Question 38 :
If n ∈ N, prove that, 1/log₂x + 1/log₃x + 1/log₄x + ........ 1/logₙx = 1/log_{ n! }
Answer :
Refer to attachment
Question 38 :
If a² + b² = 7ab, prove that
log[ 1/3 ( a + b ) ] = 1/2 [ log a + log b ]
Answer :
Given :
a² + b² = 7ab
Adding 2ab on both sides
⇒ a² + b² + 2ab = 7ab + 2ab
⇒ ( a + b )² = 9ab
⇒ ( a + b )² / 9 = ab
⇒ ( a + b )² / 3² = ab
⇒ [ ( a + b ) / 3 ]² = ab
Taking log on both sides
⇒ log[ 1/3( a + b ) ]² = log ab
⇒ 2log[ 1/3( a + b ) ] = log a + log b
⇒ log[ 1/3( a + b ) ] = 1/2( log a + log b )
Hence proved.
Question 40 :
If a, b, c are in GP, prove that
(i) logₓa, logₓ b, logₓ c are in AP
(ii) log_a x, log_b x, log_c x are in HP
Answer :
Refer to attachment
Question 41 :
If log₃ 2, log₃ ( 2^x - 5 ), log₃ ( 2^x - 7/2 ) are in AP, find the value of x.
Answer :
Refer to attachment
Question 42 :
Prove that :
(i) log_a n / log_(ab) n = 1 + log_a b
(ii) [ log_a x. log_b x ] / [ log_a x + log_b x ] = log_(ab) x
Answer :
Refer to attachment
Question :
Find the value of x in each of the following :
(i) log_( √8 ) x = 10/3
⇒ ( √8 )^( 10/3 ) = x
⇒ ( 2³ )^( 5/3 ) = x
⇒ 2⁵ = x
⇒32 = x
(ii) log_( √8 ) x = 3^( 1/3 )
⇒( √8 )^( ∛3 ) = x
⇒ x ≈ 4.4796
(iii) ( log_e 2 )( logₓ 625 ) = ( log₁₀ 16 )( log_e 10 )
Since log_a b = log b / log a
⇒ ( log 2 / log e )( log 5^4 / log x ) = ( log 2^4 / log 10 )( log 10 / log e )
⇒ ( log 2 )( 4log 5 ) = ( 4log 2 )( log x )
⇒ log 5 = log x
⇒ x = 5
Answer:
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