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Solve the initial value problem y' = (x + y − 3)2 with y(0) = 0. = a.

1 Answer

7 votes

If the 2 meant *2 then:

Expand and move y to left side to get

y’-2*y=2*x-6.

The homog eqn is yh’-2*yh=0 so yh=k1*exp(2*x) by trying y=exp(m*x) or separating.

Assume yp=a*x+b so yp’=a then

a-2*(a*x+b)=2*x-6 or

-2*a*x+a-2*b=2*x-6 so

-2*a=2 so a=-1 and a-2*b=-6 so

-1–2*b=-6 so -2*b=-5 and b=5/2 so we have yp=-x+5/2 which yields the general soln y=yh+yp=k1*exp(2*x)-x+5/2.

For y(0)=0, we see k1+5/2=0 so k1=-5/2 and the solution is

y=5*(1-exp(2*x))/2-x.

This heads exponentially to minf for larger x.

If the 2 is ^2 then

y’=(x+y-3)^2 and let y=v-x+3 so y’=v’-1 and y’=(x+y-3)^2 becomes v’-1=v^2 or

v’=1+v^2 so separate as dv/(1+v^2)=dx and integrate to get

atan(v)=x+k2 so v=tan(x+k2)=y+x-3 so y=tan(x+k2)-x+3 and y(0)=0 becomes

0=tan(k2)+3 and tan(k2)=-3 so k2=-atan(3) which makes y=tan(x-atan(3))-x+3.

This has singularities for x=atan(3)+%pi*(2*n+1)/2 for integer

answered
User Dyllon
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