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When I solve the system of differential equations I get two solutions for each variable (See the picture). My question is how I can neglect one of the solutions at the plot. Thanks.

Code:

[![\text{Plot}\left\[\text{Evaluate}\left\[\left\{\sqrt{y(x)},z(x)\right\}\text{/.}\, \%\right\],\{x,0,60\},\text{PlotRange}\to \text{All},\text{PlotPoints}\to 200\right\] \left(\text{sol}=\text{NDSolve}\left\[\left\{3 y(x)-0.5 z(x)^2=y(x) (9 \eta  y(x)+0.5) z'(x)^2\text{/.}\, \eta \to 1,-y'(x)=-\eta  y(x) y'(x) z'(x)^2+(4 \alpha +18) \eta  y(x)^2 z'(x)^2+y(x) z'(x)^2\text{/.}\, \{\eta \to 1,\alpha \to 1\},y(0)=1,z(0)=0.2\right\},\{z,y\},\{x,0,60\}\right\]\right)][1]][1]

enter image description here enter image description here

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  • $\begingroup$ Replace the % with sol[[1]] to take the first part? Hard to check without code that can be copied. $\endgroup$ – Chris K Nov 14 '18 at 3:10
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