Sr Examen

Ecuación diferencial dy/dx=(x+y)^2-(x+y)

El profesor se sorprenderá mucho al ver tu solución correcta😉

v

Para el problema de Cauchy:

y() =
y'() =
y''() =
y'''() =
y''''() =

Gráfico:

interior superior

Solución

Ha introducido [src]
d                    2           
--(y(x)) = (x + y(x))  - x - y(x)
dx                               
$$\frac{d}{d x} y{\left(x \right)} = - x + \left(x + y{\left(x \right)}\right)^{2} - y{\left(x \right)}$$
y' = -x + (x + y)^2 - y
Solución detallada
Tenemos la ecuación:
$$x - \left(x + y{\left(x \right)}\right)^{2} + y{\left(x \right)} + \frac{d}{d x} y{\left(x \right)} = 0$$
Sustituimos
$$u{\left(x \right)} = x + y{\left(x \right)}$$
y porque
$$\frac{d}{d x} y{\left(x \right)} + 1 = \frac{d}{d x} u{\left(x \right)}$$
entonces
$$\frac{d}{d x} y{\left(x \right)} = \frac{d}{d x} u{\left(x \right)} - 1$$
sustituimos
$$- x^{2} - 2 x \left(- x + u{\left(x \right)}\right) - \left(- x + u{\left(x \right)}\right)^{2} + u{\left(x \right)} + \frac{d}{d x} \left(- x + u{\left(x \right)}\right) = 0$$
o
$$- u^{2}{\left(x \right)} + u{\left(x \right)} + \frac{d}{d x} u{\left(x \right)} - 1 = 0$$
Esta ecuación diferencial tiene la forma:
f1(x)*g1(u)*u' = f2(x)*g2(u),

donde
$$\operatorname{f_{1}}{\left(x \right)} = 1$$
$$\operatorname{g_{1}}{\left(u \right)} = 1$$
$$\operatorname{f_{2}}{\left(x \right)} = -1$$
$$\operatorname{g_{2}}{\left(u \right)} = - u^{2}{\left(x \right)} + u{\left(x \right)} - 1$$
Pasemos la ecuación a la forma:
g1(u)/g2(u)*u'= f2(x)/f1(x).

Dividamos ambos miembros de la ecuación en g2(u)
$$- u^{2}{\left(x \right)} + u{\left(x \right)} - 1$$
obtendremos
$$- \frac{\frac{d}{d x} u{\left(x \right)}}{u^{2}{\left(x \right)} - u{\left(x \right)} + 1} = -1$$
Con esto hemos separado las variables x y u.

Ahora multipliquemos las dos partes de la ecuación por dx,
entonces la ecuación será así
$$- \frac{dx \frac{d}{d x} u{\left(x \right)}}{u^{2}{\left(x \right)} - u{\left(x \right)} + 1} = - dx$$
o
$$- \frac{du}{u^{2}{\left(x \right)} - u{\left(x \right)} + 1} = - dx$$

Tomemos la integral de las dos partes de la ecuación:
- de la parte izquierda la integral por u,
- de la parte derecha la integral por x.
$$\int \left(- \frac{1}{u^{2} - u + 1}\right)\, du = \int \left(-1\right)\, dx$$
Solución detallada de la integral con u
Solución detallada de la integral con x
Tomemos estas integrales
$$- \frac{2 \sqrt{3} \operatorname{atan}{\left(\frac{2 \sqrt{3} u}{3} - \frac{\sqrt{3}}{3} \right)}}{3} = Const - x$$
Solución detallada de una ecuación simple
Hemos recibido una ecuación ordinaria con la incógnica u.
(Const - es una constante)

La solución:
$$\operatorname{u_{1}} = u{\left(x \right)} = - \frac{\sqrt{3} \tan{\left(C_{1} - \frac{\sqrt{3} x}{2} \right)}}{2} + \frac{1}{2}$$
hacemos cambio inverso
$$y{\left(x \right)} = - x + u{\left(x \right)}$$
$$y1 = y(x) = - x - \frac{\sqrt{3} \tan{\left(C_{1} - \frac{\sqrt{3} x}{2} \right)}}{2} + \frac{1}{2}$$
Respuesta [src]
             2                                           3 /               2              2                /               2              2\\    4 /                         /               2              2\                /                         /               2              2\                             \                             \    5 /                           2        2               /                         /               2              2\                             \                /                           2        2               /                         /               2              2\                             \                  /                        2       2\\                  /                        2       2\\                         
            x *(-1 + 2*C1 + C1*(-1 + C1)*(-1 + 2*C1))   x *\2 + (-1 + 2*C1)  - 2*C1 + 2*C1  + C1*(-1 + C1)*\2 + (-1 + 2*C1)  - 2*C1 + 2*C1 //   x *\-6 + 12*C1 + (-1 + 2*C1)*\2 + (-1 + 2*C1)  - 2*C1 + 2*C1 / + C1*(-1 + C1)*\-6 + 12*C1 + (-1 + 2*C1)*\2 + (-1 + 2*C1)  - 2*C1 + 2*C1 / + 6*C1*(-1 + C1)*(-1 + 2*C1)/ + 6*C1*(-1 + C1)*(-1 + 2*C1)/   x *\16 - 16*C1 + 14*(-1 + 2*C1)  + 16*C1  + (-1 + 2*C1)*\-6 + 12*C1 + (-1 + 2*C1)*\2 + (-1 + 2*C1)  - 2*C1 + 2*C1 / + 6*C1*(-1 + C1)*(-1 + 2*C1)/ + C1*(-1 + C1)*\16 - 16*C1 + 14*(-1 + 2*C1)  + 16*C1  + (-1 + 2*C1)*\-6 + 12*C1 + (-1 + 2*C1)*\2 + (-1 + 2*C1)  - 2*C1 + 2*C1 / + 6*C1*(-1 + C1)*(-1 + 2*C1)/ + 2*C1*(-1 + C1)*\8 - 8*C1 + 7*(-1 + 2*C1)  + 8*C1 // + 2*C1*(-1 + C1)*\8 - 8*C1 + 7*(-1 + 2*C1)  + 8*C1 //                     / 6\
y(x) = C1 + ----------------------------------------- + ------------------------------------------------------------------------------------- + ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- + --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- + C1*x*(-1 + C1) + O\x /
                                2                                                                 6                                                                                                                                               24                                                                                                                                                                                                                                                                                                                120                                                                                                                                                                                                                                     
$$y{\left(x \right)} = \frac{x^{2} \left(C_{1} \left(C_{1} - 1\right) \left(2 C_{1} - 1\right) + 2 C_{1} - 1\right)}{2} + \frac{x^{3} \left(2 C_{1}^{2} + C_{1} \left(C_{1} - 1\right) \left(2 C_{1}^{2} - 2 C_{1} + \left(2 C_{1} - 1\right)^{2} + 2\right) - 2 C_{1} + \left(2 C_{1} - 1\right)^{2} + 2\right)}{6} + \frac{x^{4} \left(6 C_{1} \left(C_{1} - 1\right) \left(2 C_{1} - 1\right) + C_{1} \left(C_{1} - 1\right) \left(6 C_{1} \left(C_{1} - 1\right) \left(2 C_{1} - 1\right) + 12 C_{1} + \left(2 C_{1} - 1\right) \left(2 C_{1}^{2} - 2 C_{1} + \left(2 C_{1} - 1\right)^{2} + 2\right) - 6\right) + 12 C_{1} + \left(2 C_{1} - 1\right) \left(2 C_{1}^{2} - 2 C_{1} + \left(2 C_{1} - 1\right)^{2} + 2\right) - 6\right)}{24} + \frac{x^{5} \left(16 C_{1}^{2} + 2 C_{1} \left(C_{1} - 1\right) \left(8 C_{1}^{2} - 8 C_{1} + 7 \left(2 C_{1} - 1\right)^{2} + 8\right) + C_{1} \left(C_{1} - 1\right) \left(16 C_{1}^{2} + 2 C_{1} \left(C_{1} - 1\right) \left(8 C_{1}^{2} - 8 C_{1} + 7 \left(2 C_{1} - 1\right)^{2} + 8\right) - 16 C_{1} + 14 \left(2 C_{1} - 1\right)^{2} + \left(2 C_{1} - 1\right) \left(6 C_{1} \left(C_{1} - 1\right) \left(2 C_{1} - 1\right) + 12 C_{1} + \left(2 C_{1} - 1\right) \left(2 C_{1}^{2} - 2 C_{1} + \left(2 C_{1} - 1\right)^{2} + 2\right) - 6\right) + 16\right) - 16 C_{1} + 14 \left(2 C_{1} - 1\right)^{2} + \left(2 C_{1} - 1\right) \left(6 C_{1} \left(C_{1} - 1\right) \left(2 C_{1} - 1\right) + 12 C_{1} + \left(2 C_{1} - 1\right) \left(2 C_{1}^{2} - 2 C_{1} + \left(2 C_{1} - 1\right)^{2} + 2\right) - 6\right) + 16\right)}{120} + C_{1} + C_{1} x \left(C_{1} - 1\right) + O\left(x^{6}\right)$$
Gráfico para el problema de Cauchy
Clasificación
1st power series
lie group
Respuesta numérica [src]
(x, y):
(-10.0, 0.75)
(-7.777777777777778, 8.68791671309618)
(-5.555555555555555, 306933482180.51166)
(-3.333333333333333, nan)
(-1.1111111111111107, 2.78363573e-315)
(1.1111111111111107, 8.427456047434801e+197)
(3.333333333333334, 3.1933833808213433e-248)
(5.555555555555557, 3.4667248631491264e+179)
(7.777777777777779, 8.388243566957884e+296)
(10.0, 3.861029683e-315)
(10.0, 3.861029683e-315)