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单词 ExampleOfDerivativeAsParameter
释义

example of derivative as parameter


For solving the (nonlinear) differential equationMathworldPlanetmath

x=y3p-2py2(1)

with  p=dydx,  according to III in the parent entry (http://planetmath.org/DerivativeAsParameterForSolvingDifferentialEquations), we differentiate both sides in regard to y, getting first

1p=13p-(y3p2+2y2)dpdy-4py.

Removing the denominators, we obtain

2p+(y+6p2y2)dpdy+12p3y=0.

The left hand side can be factored:

(ydpdy+2p)(1+6p2y)=0(2)

Now we may use the zero rule of product; the first factor of the product in (2) yields  ydpdy=-2p, i.e.

2dyy=-dpp+lnC,

whence  y2=Cp,  i.e.  p=Cy2.  Substituting this into the original equation (1) we get  x=y33C-2C.  Hence the general solution of (1) may be written

y3=3Cx+6C2.

The second factor in (2) yields  6p2y=-1,  which is substituted into (1) multiplied by 3p:

3px=y-(-y)

Thus we see that  p=2y3x, which is again set into (1), giving

x=y3x32y-4y33x.

Finally, we can write it

3x2=-8y3,

which (a variant of the so-called semicubical parabola) is the singular solution of (1).

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更新时间:2025/5/3 18:29:25