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I have two differential equations: $da/dt = a (.3 a^{-3} + .7)^{1/2}$ and $d \tau /dt = 1/a$. The initial conditions are $t = 0$; $a = 1$ and $\tau = 0$, respectively.

How can I solve the equations in Mathematica? I need to calculate different values of $a$, $t$ and $\tau$ also plot $\tau$ vs $a$.

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  • $\begingroup$ Thanks a lot, @David :) $\endgroup$
    – Photon
    Nov 28, 2018 at 8:36

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How can I solve the equations in Mathematica

One way could be to use NDSolve

ClearAll[a,t,tau];
ode1   = a'[t]   == a[t] Sqrt[ (3/10  * 1/a[t]^3 + 7/10)];
ode2   = tau'[t] == 1/a[t];
ic1    = a[0]    == 1;
ic2    = tau[0]  == 0;
sol    = NDSolve[{ode1,ode2,ic1,ic2},{a[t],tau[t]},{t,0,5}]

Mathematica graphics

And now you can plot them

 Plot[Evaluate[a[t] /. sol], {t, 0, 5}, PlotRange -> All]

Mathematica graphics

 Plot[Evaluate[tau[t] /. sol], {t, 0, 5}, PlotRange -> All]

Mathematica graphics

Is it possible to plot a[t] vs /tau?

You can try to use ParametricPlot for this. 

 ParametricPlot[{Evaluate[a[t]], Evaluate[tau[t]]} /. sol, {t, 0, 0.5}]

Mathematica graphics

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  • $\begingroup$ Thanks a lot for your effort. But still now I found error when trying to draw a[t] vs /tau. Can you please resolve this? $\endgroup$
    – Photon
    Nov 28, 2018 at 9:37
  • $\begingroup$ Is it possible to plot a[t] vs /tau? $\endgroup$
    – Photon
    Nov 28, 2018 at 10:18
  • $\begingroup$ @Photon try ParametricPlot to plot a(t) vs. tau(t) $\endgroup$
    – Nasser
    Nov 28, 2018 at 16:47

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