# plotting Integrating by using numerical solution

pde1 = -y1''[x] - (2*y1'[x])/x + ((y1[x])^3 + y2[x])y1[x] == 0;
pde2 = y2''[x] + (2y2'[x])/x - (y1[x])^3 == 0;
sol = NDSolve[ {pde1, pde2, y1 == 0.001, y2 == -0.001,
y1'[0.001] == 0.001, y2'[0.001] == 0.001}, {y1, y2}, {x,0.001, 20}]


I need to plot the values of Integrate[ y2[x]*y1[x]^3* x^2,{x,0.001, 20}]

• {fy1, fy2} = Values@First@sol; NIntegrate[fy2[x]*fy1[x]^3 x^2, {x, 0.001, 20}] .What you're asking for gives a single number so I don't know why you're asking for a plot. Dec 2 '20 at 18:52
• thank you so much for your answer , i was think it can varies in the interval but i'm not sure Dec 2 '20 at 19:32
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You can add a extra differential equation f'[x]==y2[x]*y1[x]^3* x^2 in your equations and solve f'[x] at the same time.

so we need to set the initial value f[0.001]=c and c is a parameters,so we use ParametricNDSolve

Clear[pde1, pde2, pde3, sol];
pde1 = -y1''[x] - (2*y1'[x])/x + ((y1[x])^3 + y2[x]) y1[x] == 0;
pde2 = y2''[x] + (2 y2'[x])/x - (y1[x])^3 == 0;
pde3 = f'[x] == y2[x]*y1[x]^3*x^2;
sol = ParametricNDSolve[{pde1, pde2, pde3, y1 == 0.001,
y2 == -0.001, y1'[0.001] == 0.001, y2'[0.001] == 0.001,
f == c}, {y1, y2, f}, {x, 0.001, 20}, {c}]
f - f[0.001] /. sol
f - f[0.001] /. sol
Plot[f[x] /. sol, {x, 0.001, 20}]
Plot[f[x] /. sol, {x, 0.001, 20}]


-2.38247*10^-9

-2.3811*10^-9

• Thank you so much Dec 3 '20 at 16:48
• This method is relatively inaccurate because it depends on the errors at only two points. With the method, @flinty proposed, errors are averaged over the whole integration interval. The relative error here is 0.6 percent, whereas the method of flinty calculated with machinePrecision gives an error of about 0.002 percent. Dec 4 '20 at 7:01
• The main reason the result is inaccurate is that the default AccuracyGoal is almost 8 dec. pl. and the result is 2*10^-9 — that is, the goal is to get at least one digit of accuracy. Use AccuracyGoal -> 16 or 17 (== - Log10[result] + MachinePrecision/2), and this result will be equivalent to flinty's. A minor reason is a typo in the initial condition: Use f[0.001] == c. Then f gives the most accurate result. The number f - 1 is less accurate due to subtractive cancellation. (I think the parameter c is useless in this case for the OP's particular problem.) Jan 2 at 16:00