Cómo calcular el coeficiente de [math] \ displaystyle x ^ {n} [/ math] en la expansión de [math] \ displaystyle \ frac {a + bx + cx ^ {2}} {e ^ {x}} [ /matemáticas]

Dado, [matemáticas] \ begin {align} \ frac {a + bx + cx ^ {2}} {e ^ {x}} & = \ left (a + bx + cx ^ {2} \ right) e ^ { -x} \\ & = \ left (a + bx + cx ^ {2} \ right) \ sum_ {n = 0} ^ {\ infty} \ frac {(- 1) ^ {n} x ^ {n} } {n!} \\ & = a \ sum_ {n = 0} ^ {\ infty} \ frac {(- 1) ^ {n} x ^ {n}} {n!} + b \ sum_ {n = 0} ^ {\ infty} \ frac {(- 1) ^ {n} x ^ {n + 1}} {n!} + C \ sum_ {n = 0} ^ {\ infty} \ frac {(- 1 ) ^ {n} x ^ {n + 2}} {n!} \ end {align} [/ math]

Por lo tanto, el coeficiente de [math] \ displaystyle x ^ {n} [/ math] en la expansión de [math] \ displaystyle \ left (\ frac {a + bx + cx ^ {2}} {e ^ {x}} \ right) [/ math]

[matemáticas] \ begin {align} & = \ frac {a (-1) ^ {n}} {n!} + \ frac {b (-1) ^ {n-1}} {(n-1)! } + \ frac {c (-1) ^ {n-2}} {(n-2)!} \\ & = \ frac {(- 1) ^ {n}} {n!} \ left [a- bn + cn (n-1) \ right] \\ & = \ frac {(- 1) ^ {n}} {n!} \ left [cn ^ {2} – (b + c) n + a \ right ] \;. \ end {align} [/ math]

Shukriya 🙂