[Math] Laplace Transform with initial value

laplace transformordinary differential equations

Use the Laplace transform to solve the following initial value
problem:

$$y'' + y = 2t$$

with $y(\pi/4) = \pi / 2 $ and $y'(\pi/4) = 2 – \sqrt{2}$.

I understand this type of problems but with initial values for $y(0)$ and $y'(0)$ .. How could I solve it with $y(\pi /4)$ and $y'(\pi /4)$ ?

Best Answer

Laplace-transforming both sides of $y'' + y = 2t$,

$$s^2 Y (s) - y_0 s - v_0 + Y (s) = \frac{2}{s^2}$$

Hence,

$$Y (s) = y_0 \left(\frac{s}{s^2 + 1}\right) + (v_0 - 2)\left(\frac{1}{s^2 + 1}\right) + \frac{2}{s^2}$$

Taking the inverse Laplace transform,

$$y (t) = y_0 \cos (t) + (v_0 - 2) \sin (t) + 2 t$$

From the conditions $y(\pi /4) = \pi / 2 $ and $y'(\pi /4) = 2- \sqrt{2}$, we get

$$y_0 + v_0 = 2 \qquad \qquad \qquad y_0 - v_0 = 2$$

Thus, $y_0 = 2$ and $v_0 = 0$, and

$$y (t) = 2 \cos (t) - 2 \sin (t) + 2 t$$