Hi,
I have to solve:
1. y''+4y'+15y=0
Multiplying through used y=e^(landa x):
Gives essentially the quadratic:
y^2+4y+25=0
This gives two complex roots being from what I can see -2 plus or minus i/2sqrt 84.
Solving the DE then gives y(x)=e^-1/2x(Acos(1/2sqrt84x+Bsin1/2sqrt84x)
Does that...