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Web skærmformat. - Aarhus Universitet

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I. Differentiation – 2. Partielle afledede 51<br />

Eksempel 9<br />

Funktionen u(t,x) = sin(x − at) er løsning til bølgeligningen<br />

Løsning<br />

giver<br />

utt = a 2 uxx<br />

ut = −acos(x − at), utt = −a 2 sin(x − at)<br />

ux = cos(x − at), uxx = −sin(x − at)<br />

utt = a 2 uxx<br />

2.31. Test Laplaces ligning ☞ [S] 11.3 Partial derivatives<br />

Test<br />

Funktionen f(x,y) = 3x + 5y + 10 er en løsning til Laplace’s ligning<br />

Løsning<br />

Udregningen<br />

giver<br />

∂ 2 f/∂x 2 + ∂ 2 f/∂y 2 = 0.<br />

fx = 3, fxx = 0, fy = 5, fyy = 0<br />

fxx + fyy = 0<br />

Afkryds:<br />

ja nej

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