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Web skærmformat. - Aarhus Universitet

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III. Potensrækker – 3. Potensrækker 297<br />

Eksempel 1<br />

Vi søger en løsning af form<br />

y ′′ + y = 0<br />

y(x) = c0 + c1x + c2x 2 + c3x 3 + ...<br />

y ′ (x) = c1 + 2 · c2x + 3 · c3x 2 + 4 · c4x 3 + ...<br />

y ′′ (x) = 2 · c2 + 3 · 2 · c3x + 4 · 3 · c4x 2 + 5 · 4 · c5x 3 + ...<br />

3.36. Potensrækkeløsning ☞ [S] 8.10 Using . . . diff. eq.<br />

Eksempel 1 - fortsat<br />

y(x) + y ′′ (x) = (c0 + 2c2) + (c1 + 3 · 2 · c3)x + (c2 + 4 · 3 · c4)x 2 + ...<br />

Fra y + y ′′ = 0 fås at alle koefficienterne må være 0, altså<br />

c0 + 2c2 = 0<br />

c1 + 3 · 2c3 = 0<br />

c2 + 4 · 3c4 = 0<br />

3.37. Potensrækkeløsning ☞ [S] 8.10 Using . . . diff. eq.

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