求二阶微分方程
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求二阶微分方程
求二阶微分方程
求二阶微分方程
特征方程:λ² - 5λ + 6 =0
λ=2,λ=3
相应的齐次方程有通
C₁e^(2x) + C₂e^(3x)
设原方程有特解y= (A₁x +A₂x²)*e^(3x)
y' = (A₁+ 2A₂x) e^(3x) + (A₁x+A₂x²)*3e^(3x)
= [A₁+(3A₁+2A₂)x +3A₂x²] e^(3x)
y'' = (3A₁+2A₂+6A₂x) e^(3x) + [A₁+(3A₁+2A₂)x +3A₂x²] *3e^(3x)
= [ 6A₁+2A₂+(9A₁+12A₂)x +9A₂x² ] e^(3x)
代入原方程:
[ 6A₁+2A₂+(9A₁+12A₂)x +9A₂x² ] - 5 [A₁+(3A₁+2A₂)x +3A₂x²] + 6 (A₁x +A₂x²) = x
【e^(3x)约掉了】
整理得:A₁+2A₂ +2A₂x = x
∴A₁+2A₂ =0 2A₂=1
A₁ = -1 A₂ = 1/2
即原方程有特y = (x²/2 - x) e^(3x)
故原方程的通解为:y = (x²/2 - x) e^(3x) + C₁e^(2x) + C₂e^(3x)