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

17.2  a) solve y'-3y/x-x=0
      b) solve y'-y=x^2
      d) solve y'-y*cos(x)=x^2*e^(sin x)
      e) solve (1+x^2)*y' = 2xy
      f) solve xy'-y+x^2=0
      g) solve y'-y=x-1
      h) solve xe^xy'+ye^x=1
      i) solve y' = (y+1)/x
      p) solve xy'+y-e^x=0, y(a)=b
      q) solve xy'+y/(x+1)=x, y(1)=0