Theorem relogdiv

Description: The natural logarithm of the quotient of two positive real numbers is the difference of natural logarithms. Exercise 72(a) and Property 3 of Cohen p. 301, restricted to natural logarithms. (Contributed by Steve Rodriguez, 25-Nov-2007)

	Ref	Expression
Assertion	relogdiv	⊢ ( ( 𝐴 ∈ ℝ+ ∧ 𝐵 ∈ ℝ+ ) → ( log ‘ ( 𝐴 / 𝐵 ) ) = ( ( log ‘ 𝐴 ) − ( log ‘ 𝐵 ) ) )

Proof

Step	Hyp	Ref	Expression
1		efsub	⊢ ( ( ( log ‘ 𝐴 ) ∈ ℂ ∧ ( log ‘ 𝐵 ) ∈ ℂ ) → ( exp ‘ ( ( log ‘ 𝐴 ) − ( log ‘ 𝐵 ) ) ) = ( ( exp ‘ ( log ‘ 𝐴 ) ) / ( exp ‘ ( log ‘ 𝐵 ) ) ) )
2		resubcl	⊢ ( ( ( log ‘ 𝐴 ) ∈ ℝ ∧ ( log ‘ 𝐵 ) ∈ ℝ ) → ( ( log ‘ 𝐴 ) − ( log ‘ 𝐵 ) ) ∈ ℝ )
3	1 2	relogoprlem	⊢ ( ( 𝐴 ∈ ℝ+ ∧ 𝐵 ∈ ℝ+ ) → ( log ‘ ( 𝐴 / 𝐵 ) ) = ( ( log ‘ 𝐴 ) − ( log ‘ 𝐵 ) ) )