Sr Examen

Ecuación diferencial dy/dx=cos(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                       
--(y(x)) = cos(x - y(x))
dx                      
$$\frac{d}{d x} y{\left(x \right)} = \cos{\left(x - y{\left(x \right)} \right)}$$
y' = cos(x - y)
Solución detallada
Tenemos la ecuación:
$$- \cos{\left(x - y{\left(x \right)} \right)} + \frac{d}{d x} y{\left(x \right)} = 0$$
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
$$- \cos{\left(u{\left(x \right)} \right)} + \frac{d}{d x} \left(x + u{\left(x \right)}\right) = 0$$
o
$$- \cos{\left(u{\left(x \right)} \right)} + \frac{d}{d x} u{\left(x \right)} + 1 = 0$$
Esta ecuación diferencial tiene la forma:
f1(x)*g1(u)*u' = f2(x)*g2(u),

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

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

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

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.
$$\int \left(- \frac{1}{\cos{\left(u \right)} - 1}\right)\, du = \int \left(-1\right)\, dx$$
Solución detallada de la integral con u
Solución detallada de la integral con x
Tomemos estas integrales
$$- \frac{1}{\tan{\left(\frac{u}{2} \right)}} = Const - x$$
Solución detallada de una ecuación simple
Hemos recibido una ecuación ordinaria con la incógnica u.
(Const - es una constante)

La solución:
$$\operatorname{u_{1}} = u{\left(x \right)} = - 2 \operatorname{atan}{\left(\frac{1}{C_{1} - x} \right)}$$
hacemos cambio inverso
$$y{\left(x \right)} = x + u{\left(x \right)}$$
$$y1 = y(x) = x - 2 \operatorname{atan}{\left(\frac{1}{C_{1} - x} \right)}$$
Respuesta [src]
                         3 /   2          2                 /   2          2              \        \    5 /       2            2          2     /       2          2                                           \   /                2            2    \           /                2            2       /       2            2              \           /   2          2              \                2                     \           /                2            2          2                        /   2          2              \           /       2            2              \           /       2            2          2                       /   2          2              \           /                2            2    \              2                               \              2                          2     /       2          2                                           \\           /   2          2              \                2                              \    2                          4 /        2          2                                             /       2          2                                           \        \                
                        x *\cos (C1) - sin (C1) - cos(C1) + \sin (C1) - cos (C1) + cos(C1)/*cos(C1)/   x *\- 4*cos (C1) + 4*sin (C1) + sin (C1)*\1 + cos (C1) - sin (C1) - 5*cos(C1) + (-1 + 4*cos(C1))*cos(C1)/ + \-cos(C1) - 4*sin (C1) + 4*cos (C1)/*cos(C1) + \-cos(C1) - 4*sin (C1) + 4*cos (C1) + \- 4*cos (C1) + 4*sin (C1) + cos(C1)/*cos(C1) - \cos (C1) - sin (C1) - cos(C1)/*cos(C1) + 2*sin (C1)*(-1 + 4*cos(C1))/*cos(C1) + \-cos(C1) - 4*sin (C1) + 4*cos (C1) + sin (C1)*(-1 + 4*cos(C1)) + \sin (C1) - cos (C1) + cos(C1)/*cos(C1) + \- 4*cos (C1) + 4*sin (C1) + cos(C1)/*cos(C1) + \- 4*cos (C1) + 4*sin (C1) + sin (C1)*(1 - 4*cos(C1)) + \cos (C1) - sin (C1) - cos(C1)/*cos(C1) + \-cos(C1) - 4*sin (C1) + 4*cos (C1)/*cos(C1) - sin (C1)*(-1 + 4*cos(C1)) + cos(C1)/*cos(C1) - sin (C1)*(1 - 4*cos(C1)) - sin (C1)*\1 + cos (C1) - sin (C1) - 5*cos(C1) + (-1 + 4*cos(C1))*cos(C1)//*cos(C1) - \sin (C1) - cos (C1) + cos(C1)/*cos(C1) + 2*sin (C1)*(1 - 4*cos(C1)) + cos(C1)/   x *(1 - cos(C1))*sin(C1)   x *\-1 + sin (C1) - cos (C1) + 5*cos(C1) + (1 - 4*cos(C1))*cos(C1) + \1 + cos (C1) - sin (C1) - 5*cos(C1) + (-1 + 4*cos(C1))*cos(C1)/*cos(C1)/*sin(C1)    / 6\
y(x) = C1 + x*cos(C1) + ---------------------------------------------------------------------------- + --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- + ------------------------ + ------------------------------------------------------------------------------------------------------------------------------------------------------ + O\x /
                                                             6                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      120                                                                                                                                                                                                                                                                                                                                                                                                                                                           2                                                                                         24                                                                                  
$$y{\left(x \right)} = x \cos{\left(C_{1} \right)} + \frac{x^{2} \left(1 - \cos{\left(C_{1} \right)}\right) \sin{\left(C_{1} \right)}}{2} + \frac{x^{3} \left(\left(\sin^{2}{\left(C_{1} \right)} - \cos^{2}{\left(C_{1} \right)} + \cos{\left(C_{1} \right)}\right) \cos{\left(C_{1} \right)} - \sin^{2}{\left(C_{1} \right)} + \cos^{2}{\left(C_{1} \right)} - \cos{\left(C_{1} \right)}\right)}{6} + \frac{x^{4} \left(\left(1 - 4 \cos{\left(C_{1} \right)}\right) \cos{\left(C_{1} \right)} + \left(\left(4 \cos{\left(C_{1} \right)} - 1\right) \cos{\left(C_{1} \right)} - \sin^{2}{\left(C_{1} \right)} + \cos^{2}{\left(C_{1} \right)} - 5 \cos{\left(C_{1} \right)} + 1\right) \cos{\left(C_{1} \right)} + \sin^{2}{\left(C_{1} \right)} - \cos^{2}{\left(C_{1} \right)} + 5 \cos{\left(C_{1} \right)} - 1\right) \sin{\left(C_{1} \right)}}{24} + \frac{x^{5} \left(2 \left(1 - 4 \cos{\left(C_{1} \right)}\right) \sin^{2}{\left(C_{1} \right)} + \left(- 4 \sin^{2}{\left(C_{1} \right)} + 4 \cos^{2}{\left(C_{1} \right)} - \cos{\left(C_{1} \right)}\right) \cos{\left(C_{1} \right)} - \left(\sin^{2}{\left(C_{1} \right)} - \cos^{2}{\left(C_{1} \right)} + \cos{\left(C_{1} \right)}\right) \cos{\left(C_{1} \right)} + \left(\left(4 \cos{\left(C_{1} \right)} - 1\right) \cos{\left(C_{1} \right)} - \sin^{2}{\left(C_{1} \right)} + \cos^{2}{\left(C_{1} \right)} - 5 \cos{\left(C_{1} \right)} + 1\right) \sin^{2}{\left(C_{1} \right)} + \left(2 \left(4 \cos{\left(C_{1} \right)} - 1\right) \sin^{2}{\left(C_{1} \right)} - \left(- \sin^{2}{\left(C_{1} \right)} + \cos^{2}{\left(C_{1} \right)} - \cos{\left(C_{1} \right)}\right) \cos{\left(C_{1} \right)} + \left(4 \sin^{2}{\left(C_{1} \right)} - 4 \cos^{2}{\left(C_{1} \right)} + \cos{\left(C_{1} \right)}\right) \cos{\left(C_{1} \right)} - 4 \sin^{2}{\left(C_{1} \right)} + 4 \cos^{2}{\left(C_{1} \right)} - \cos{\left(C_{1} \right)}\right) \cos{\left(C_{1} \right)} + \left(- \left(1 - 4 \cos{\left(C_{1} \right)}\right) \sin^{2}{\left(C_{1} \right)} + \left(4 \cos{\left(C_{1} \right)} - 1\right) \sin^{2}{\left(C_{1} \right)} + \left(\sin^{2}{\left(C_{1} \right)} - \cos^{2}{\left(C_{1} \right)} + \cos{\left(C_{1} \right)}\right) \cos{\left(C_{1} \right)} + \left(4 \sin^{2}{\left(C_{1} \right)} - 4 \cos^{2}{\left(C_{1} \right)} + \cos{\left(C_{1} \right)}\right) \cos{\left(C_{1} \right)} - \left(\left(4 \cos{\left(C_{1} \right)} - 1\right) \cos{\left(C_{1} \right)} - \sin^{2}{\left(C_{1} \right)} + \cos^{2}{\left(C_{1} \right)} - 5 \cos{\left(C_{1} \right)} + 1\right) \sin^{2}{\left(C_{1} \right)} + \left(\left(1 - 4 \cos{\left(C_{1} \right)}\right) \sin^{2}{\left(C_{1} \right)} - \left(4 \cos{\left(C_{1} \right)} - 1\right) \sin^{2}{\left(C_{1} \right)} + \left(- 4 \sin^{2}{\left(C_{1} \right)} + 4 \cos^{2}{\left(C_{1} \right)} - \cos{\left(C_{1} \right)}\right) \cos{\left(C_{1} \right)} + \left(- \sin^{2}{\left(C_{1} \right)} + \cos^{2}{\left(C_{1} \right)} - \cos{\left(C_{1} \right)}\right) \cos{\left(C_{1} \right)} + 4 \sin^{2}{\left(C_{1} \right)} - 4 \cos^{2}{\left(C_{1} \right)} + \cos{\left(C_{1} \right)}\right) \cos{\left(C_{1} \right)} - 4 \sin^{2}{\left(C_{1} \right)} + 4 \cos^{2}{\left(C_{1} \right)} - \cos{\left(C_{1} \right)}\right) \cos{\left(C_{1} \right)} + 4 \sin^{2}{\left(C_{1} \right)} - 4 \cos^{2}{\left(C_{1} \right)} + \cos{\left(C_{1} \right)}\right)}{120} + C_{1} + 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, -0.28186868044546876)
(-5.555555555555555, 1.2604829090120742)
(-3.333333333333333, 3.2864068173010192)
(-1.1111111111111107, 5.417486999261139)
(1.1111111111111107, 7.587290771743678)
(3.333333333333334, 9.775506932241083)
(5.555555555555557, 11.973896225436578)
(7.777777777777779, 14.178492158544433)
(10.0, 16.38715148932226)
(10.0, 16.38715148932226)