You can judge this one by its cover.

Calculus Level 5

0 1 0 1 1 + x y 2 + x y 2 + x y 2 + d y d x \displaystyle \int_{0}^1 \int_0^1 1 + \dfrac{xy}{2 + \dfrac{xy}{2+ \dfrac{xy}{2+ \cdots}}} \, dy \, dx

If the answer can be represented in the form a b [ a b + c b 2 + ln a 2 ( 1 + a 2 ) ] \dfrac{a}{b} \left[ -\dfrac{a}{b} + \dfrac{c}{b} \sqrt{2} + \ln \dfrac{a}{2\left( 1 + \sqrt{\dfrac{a}{2}} \right) } \right] , where a , b , c Z + a, b, c \in \mathbb{Z}^+ and a b \dfrac{a}{b} is in simplest form, find a + b + c a + b + c .


The answer is 12.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Mark Hennings
Dec 29, 2017

The integrand is F ( x , y ) = 1 + x y 1 + F ( x , y ) = 1 + x y F(x,y) \; = \; 1 + \frac{xy}{1 + F(x,y)} \;= \; \sqrt{1 + xy} and so the integral is I = 0 1 0 1 F ( x , y ) d x d y = 0 1 d u u 1 1 + u v d v = 0 1 ln u 1 + u d u = 2 3 0 1 ( 1 + u ) 3 2 1 u d u \begin{aligned} I & = \; \int_0^1 \int_0^1 F(x,y)\,dx\,dy \; = \; \int_0^1 \,du \int_u^1 \frac{\sqrt{1+u}}{v}\,dv \; = \; -\int_0^1 \ln u \sqrt{1+u}\,du \\ & = \; \tfrac23\int_0^1 \frac{(1+u)^{\frac32} - 1}{u}\,du \end{aligned} using first the change of variables u = x y , v = x u= xy\,,\, v = x , and then integrating by parts. Now the substitution z = 1 + u z = \sqrt{1+u} gives I = 2 3 1 2 z 3 1 z 2 1 × 2 z d z = 4 3 1 2 z 4 z z 2 1 d z = 4 3 1 2 ( z 2 + 1 1 z + 1 ) d z = 4 3 [ 1 3 z 2 + z ln ( z + 1 ) ] 1 2 = 4 3 [ 4 3 + 5 3 2 + ln 2 ln ( 1 + 2 ) ] = 4 3 [ 4 3 + 5 3 2 + ln ( 4 2 ( 1 + 4 2 ) ) ] \begin{aligned} I & = \; \tfrac23\int_1^{\sqrt{2}} \frac{z^3 - 1}{z^2-1} \times 2z\,dz \; = \; \tfrac43\int_1^{\sqrt{2}}\frac{z^4-z}{z^2-1}\,dz \\ & = \; \tfrac43\int_1^{\sqrt{2}} \left(z^2 + 1 - \frac{1}{z+1}\right)\,dz \; = \; \tfrac43\Big[\tfrac13z^2 + z - \ln(z+1)\Big]_1^{\sqrt{2}} \\ & = \; \tfrac43\Big[-\tfrac43 + \tfrac53\sqrt{2} + \ln2 - \ln(1 + \sqrt{2})\Big] \\ & = \; \tfrac43\Big[-\tfrac43 + \tfrac53\sqrt{2} + \ln\Big(\frac{4}{2\big(1 + \sqrt{\frac{4}{2}}\big)}\Big)\Big] \end{aligned} making the answer 4 + 3 + 5 = 12 4+3+5=\boxed{12} .

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...