11meters long because 24-13=11 so 11 is the answer
From Mathematica:<span>DSolve<span>[<span><span>y′</span>[x]=<span><span>(<span>y[x<span>]2</span>−<span>x2</span></span>)</span><span>2xy[x]</span></span>,y[x],x</span>]</span></span><span>{<span><span>{<span>y[x]→−<span><span>−<span>x2</span>+xC[1]</span><span>−−−−−−−−−−</span>√</span></span>}</span>,<span>{<span>y[x]→<span><span>−<span>x2</span>+xC[1]</span><span>−−−−−−−−−−</span>√</span></span>}</span></span>}</span>Take the second solution and square each side:<span>y[x<span>]2</span>=−<span>x2</span>+xC[1]</span>Move the -x^2 from the RHS to the LHS.<span>y[x<span>]2</span>+<span>x2</span>=xC[1]</span>If the same procedure is applied to my answer then the following is the result:<span>y[x<span>]2</span>+<span>x2</span>= 2cx+2C[1]</span><span>where C[1] is zero. I am not sure that their expression is totally correct.</span>
