Too many variables

Calculus Level 4

Let { f ( a , b , c ) = 0 a ( a b x b ) c d x g ( a , b , c ) = f ( a , b , c ) f ( a , b , c 1 ) \begin{cases} f(a,b,c) =\displaystyle \int_{0}^{a} (a^b-x^b)^c\, dx \\ g(a,b,c) = \dfrac {f(a,b,c)}{f(a,b,c-1)} \end{cases}

Functions f f and g g are defined as above. If g ( 5 , 6 , 104 ) = 100 K g(5,6,104) = 100K . Find K K .


The answer is 156.

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1 solution

f ( a , b , c ) = 0 a ( a b x b ) c d x f\left(a,b,c\right) = \displaystyle\int_{0}^{a}{(a^b-x^b)^c\,dx}

f ( a , b , c ) = a b 0 a ( a b x b ) c 1 d x 0 a ( x ) ( x b 1 ) ( a b x b ) c 1 d x f\left(a,b,c\right) = a^b\displaystyle\int_{0}^{a}{(a^b-x^b)^{c-1}\,dx} - \displaystyle\int_{0}^{a}{(x)(x^{b-1})(a^b-x^b)^{c-1}\,dx}

f ( a , b , c ) = a b f ( a , b , c 1 ) 0 a ( x ) ( x b 1 ) ( a b x b ) c 1 d x f\left(a,b,c\right) = a^bf\left(a,b,c-1\right) - \displaystyle\int_{0}^{a}{(x)(x^{b-1})(a^b-x^b)^{c-1}\,dx}

Consider the integral I = 0 a ( x ) ( x b 1 ) ( a b x b ) c 1 d x I =\displaystyle\int_{0}^{a}{(x)(x^{b-1})(a^b-x^b)^{c-1}\,dx}

Using integration by parts I = ( x ( a b x b ) c b c ) 0 a + 1 b c 0 a ( a b x b ) c d x I = \bigg(- \displaystyle\frac{x(a^b-x^b)^c}{bc}\bigg)\Bigg|_0^a + \displaystyle\frac{1}{bc}\displaystyle\int_{0}^{a}{(a^b-x^b)^c\,dx}

I = 0 + 1 b c f ( a , b , c ) I = 0 + \displaystyle\frac{1}{bc}f\left(a,b,c\right)

Resubstituting the value of I I we get

f ( a , b , c ) = a b f ( a , b , c 1 ) 1 b c f ( a , b , c ) f\left(a,b,c\right) = a^bf\left(a,b,c-1\right) - \displaystyle\frac{1}{bc}f\left(a,b,c\right)

Thus f ( a , b , c ) f ( a , b , c 1 ) = a b b c b c + 1 \displaystyle\frac{f\left(a,b,c\right)}{f\left(a,b,c-1\right)} = \displaystyle\frac{a^bbc}{bc+1}

Thus g ( a , b , c ) = a b b c b c + 1 g\left(a,b,c\right) = \displaystyle\frac{a^bbc}{bc+1}

Therefore g ( 5 , 6 , 104 ) = 5 6 ( 6 ) ( 104 ) ( 6 ) ( 104 ) + 1 = 15600 g\left(5,6,104\right) = \displaystyle\frac{5^6(6)(104)}{(6)(104)+1} = 15600

100 K = 15600 100K = 15600

K = 156 K=156

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