Find the particular solution of the differential equation that satisfies the initial condition(s). (Remember to use absolute values where appropriate.) f ''(x) = 4 x2 , f '(1) = 2, f(1) = 5

Respuesta :

Looks like either [tex]f''(x)=4x^2[/tex] or [tex]f''(x)=\frac4{x^2}[/tex]...

In the first case, integrate both sides twice to get

[tex]f''(x)=4x^2\implies f'(x)=\dfrac43x^3+C_1\implies f(x)=\dfrac13x^4+C_1x+C_2[/tex]

Then the initial conditions give

[tex]f'(1)=2\implies 2=\dfrac43\cdot1^3+C_1\implies C_1=\dfrac23[/tex]

[tex]f(1)=5\implies 5=\dfrac13\cdot1^4+C_1\cdot1+C_2\implies C_2=4[/tex]

so that the particular solution is

[tex]f(x)=\dfrac{x^4}3+\dfrac{2x}3+4[/tex]

If instead [tex]f''(x)=\frac4{x^2}[/tex], we have

[tex]f''(x)=\dfrac4{x^2}\implies f'(x)=-\dfrac4x+C_1\implies f(x)=-4\ln|x|+C_1x+C_2[/tex]

[tex]f'(1)=2\implies 2=-\dfrac41+C_1\implies C_1=6[/tex]

[tex]f(1)=5\implies 5=-4\ln|1|+C_1\cdot1+C_2\implies C_2=-1[/tex]

[tex]\implies f(x)=-4\ln|x|+6x-1[/tex]

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