... doubles everyhour. For the continuous problem, we assume that this i s true for y(t). We write this as an equation:y(t) = αy(t).775 1 23 4 48 12 16 1 23 4 48 12 16 Figure 14 . 1: The p opulation ... =y(x)x.P (1, u) + Q (1, u)u + xdudx= 0This equation is separable.P (1, u) + uQ (1, u) + xQ (1, u)dudx= 0 1 x+Q (1, u)P (1, u) + uQ (1, u)dudx= 0ln |x| + 1 u + P (1, u)/Q (1, u)du ... arbitraryz 1 : (1 + i)a 1 − ia 1 = 0 → a 1 = 0z2: (2 + i)a2− ia2+i2a0= 0 → a2= −i 4 a0. 811 Result 14 . 4.2 Any first order differential equation of the first degree can be written in theformP...