16.3   solve y''-3y'+2y=0

16.4   c) (c*e^x+d-x-x^2/2)''-(c*e^x+d-x-x^2/2)'-x
       e) x*(x*e^(c*x+1))'-(x*e^(c*x+1))*ln((e^(c*x+1)))
       f) (sin x -1 + c*e^(-sin x))'+(sin x -1 + c*e^(-sin x))*cos x - (sin 2x)/2

16.5   f) solve y*y'-x = 5
          plot y^2 = x^2 +10x, y^2 = x^2 +10x +1, y^2 = x^2 +10x +4, y^2 = x^2 +10x +10, y=-10..10
       g) solve x*y' = x+1
       i) solve y'=2sqrt(y)
       y=(x+1)^2, y=(x+2)^2, y=(x-3)^2 where y=-10..10

16.6   a) solve x*y'=x+1
       b) solve (x+1)*y'+(2-y)=0
       c) solve y*(y+1)*y'=1/(x-1)
       f) solve (x+1)dy + (2-y)dx =0
       g) solve(1+x^2)dy-sqrt(1-y^2)dx=0
       h) solve sqrt(1-x^2)dy-sqrt(1-y^2)dx=0

16.7   b) solve y'=e^(x+y), y(0) = 0
       d) solve y*x*y'+(x^2-1)=0, y(1)=0

16.9   solve dy/dt = k*y, y(0)=10^7
       plot 10^7*e^(2*x), 10^7*e^(-1*x)  
 
16.10  solve dy/dt = k*y
       ln(3)/ln(3/2)
 
16.13  solve dy/dx = 1/250*y(1000-y) with y(0)=2
       plot 1000*e^(4x)/(e^(4x)+499) where x = 0..10
       solve 1000*e^(4x)/(e^(4x)+499) = 500