Exponential functions!

Calculus Level 4

Let f ( x ) = 2 x f(x) = 2^x and g ( x ) = 3 x g(x) = 3^x .

f ( x ) f(x) and g ( x ) g(x) have a common tangent at A A and B B as shown above.

Find the slope of the common tangent.


The answer is 0.85740343.

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2 solutions

Barry Leung
Jan 11, 2021

Ok, this problem literally took me so long! But here it goes:

Rocco Dalto
Jan 10, 2021

Let m = ln ( 2 ) m = \ln(2) and n = ln ( 3 ) 2 = e m n = \ln(3) \implies 2 = e^m and 3 = e n 3 = e^n

f ( x ) = e m x \implies f(x) = e^{mx} and g ( x ) = e n x f ( a ) = m e m a = g ( b ) = n e n b g(x) = e^{nx} \implies f'(a) = me^{ma} = g'(b) = ne^{nb} \implies a = 1 m ln ( n m e n b ) = 1 m ln ( n m ) + n m b a = \dfrac{1}{m}\ln(\dfrac{n}{m}e^{nb}) =\dfrac{1}{m}\ln(\dfrac{n}{m}) + \dfrac{n}{m}b

\implies

A : ( 1 m ln ( n m ) + n m b , n m e n b ) A:(\dfrac{1}{m}\ln(\dfrac{n}{m}) + \dfrac{n}{m}b,\dfrac{n}{m} * e^{nb}) and B : ( b , e n b ) B:(b,e^{nb})

m A B = ( n m ) e n b ln ( n m ) + ( n m ) b = n e n b \implies m_{AB} = \dfrac{(n - m)e^{nb}}{\ln(\dfrac{n}{m}) + (n - m)b} = ne^{nb} \implies b = n m n ln ( n m ) n ( n m ) b = \dfrac{n - m - n\ln(\dfrac{n}{m})}{n(n - m)}

m A B = g ( b ) = n e ( n m ) n n m = ( m n n m ) 1 n m e \implies m_{AB} = g'(b) = \dfrac{ne}{(\dfrac{n}{m})^{\frac{n}{n - m}}} = (\dfrac{m^n}{n^m})^{\frac{1}{n - m}} * e

Replacing m = ln ( 2 ) m = \ln(2) and n = ln ( 3 ) n = \ln(3) into m A B m_{AB} we obtain:

m A B = ( ( ln ( 2 ) ) ln ( 3 ) ( ln ( 3 ) ) ln ( 2 ) ) 1 ln ( 3 2 ) e 0.85740343 m_{AB} = (\dfrac{(\ln(2))^{\ln(3)}}{(\ln(3))^{\ln(2)}})^{\dfrac{1}{\ln(\frac{3}{2})}} * e \approx \boxed{0.85740343}

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