Question #118563
Answer the questions and provide complete solution

1.5) Find the largest constant r so that phi(x) = e^rz is a solution to third derivative of y minus 2 times second derivative of y plus 2y = 0.

1.6) Find the smallest constant rso that phi(x) = x^r is a solution to (3)(x^2)(second derivative of y) + (11)(x)(first derivative of y) - 3y = 0, x > 0.
1
Expert's answer
2020-05-28T19:38:51-0400

1.5) Differential equation is y2y+2y=0y''' - 2y'' + 2y=0 and ϕ(x)=erx\phi(x)= e^{rx} is solution of this differential equation, hence ϕ(x)\phi(x) holds the differential equation.

Now,

y=ϕ(x)=erx,y=rerx,y=r2erx+erx,y=r3erx+2rerx+rerx=r3erx+3rerxy = \phi(x) = e^{rx}, y' = r e^{rx}, y'' = r^2 e^{rx} + e^{rx}, \\ y''' = r^3e^{rx} + 2re^{rx} +re^{rx} = r^3e^{rx} + 3re^{rx}.

Hence by putting in differential equation, we ger

erx(r3+3r2r22+2)=0e^{rx} (r^3+3r - 2r^2 -2 +2) = 0

Since, erx0,    r32r2+3r=0e^{rx} \neq 0, \implies r^3-2r^2 + 3r =0

    r(r22r+3)=0    r=0,r=2±4122=1±2i\implies r(r^2-2r+3) =0 \implies r=0, r = \frac{2\pm \sqrt{4-12}}{2} = 1\pm\sqrt{2} i .

Two roots are imaginary, so real largest constant is 0.


1.6) Differential equation is 3x2y+11xy3y=03x^2y''+11xy'-3y =0 .

y=xny = x^n is solution when it holds the differential equation.

Now, y=xn,y=nxn1,y=n(n1)xn2y = x^n, y' = nx^{n-1}, y'' = n(n-1) x^{n-2}

By putting, we get

    (3n(n1)+11n3)xn=0    3n2+8n3=0So, 3n2+9nn3=0    (3n1)(n+3)=0    n=13,3.\implies (3n(n-1)+11n-3)x^n=0 \implies 3n^2 +8n-3=0 \\ So, \ 3n^2+9n-n-3=0 \implies (3n-1)(n+3)=0 \\ \implies n = \frac{1}{3}, -3.

Smallest n is -3.


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