Second problem

Algebra Level 3

{ x + y = 4 x x + y y = 64 \large \begin{cases} x+y = 4 \\ x^x+y^y = 64\end{cases}

Are there real value pairs ( x , y ) (x,y) satisfying the system of equations above?

Yes No

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.

2 solutions

Ram Sita
Oct 24, 2017

We have inequality of form x^y + y^x >= 1 for positive reals x and y so use inequality to get answer

What is x with an arrow pointing up? What does "reals" mean? What is meant by "inequality"? Not familiar with any of this.

John King - 2 years, 6 months ago
Chew-Seong Cheong
Oct 22, 2017

Let f ( x ) = x x + y y f(x) = x^x + y^y , where x + y = 4 x+y=4 . Then f ( x ) = x x + ( 4 x ) 4 x f(x) = x^x + (4-x)^{4-x} and f ( x ) = ( ln x + 1 ) x x ( ln ( 4 x ) + 1 ) ( 4 x ) 4 x f'(x) = (\ln x +1)x^x - (\ln(4-x)+1)(4-x)^{4-x} . We note that f ( x ) > 0 f'(x) > 0 or f ( x ) f(x) is increasing for x ( 3 , 4 ) x \in (3,4) . Now, note that f ( 3 ) = 3 3 + 1 1 = 28 f(3) = 3^3 + 1^1 = 28 and f ( 3.4 ) = 3. 4 3.4 + 0. 6 0.6 64.861 f(3.4) = 3.4^{3.4} + 0.6^{0.6} \approx 64.861 . Implying that f ( x ) = 64 f(x) = 64 for 3 < x < 3.4 3 < x < 3.4 . In fact f ( x ) = 64 f(x)=64 when x 3.393901832 x \approx 3.393901832 and y 0.606098169 y \approx 0.606098169 . Since x x and y y are identical in the system of equations. x 0.606098169 x \approx 0.606098169 and y 3.393901832 y \approx 3.393901832 is a also a solution pair.

Answer: Yes there are real value pairs ( x , y ) (x,y) satisfying the system of equations.

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...