In Problems 13 through 18, use a computer system or graphing calculator to plot a slope field and/or enough solution curves to indicate the stability or instability of each critical point of the given differential equation. (Some of these critical points may be semistable in the sense mentioned in Example 6.) 13. dx (x + 2) (x - 2)2 dx 14. = x(12 4) dt dx dx 15. 4)2 10. di dx 17. 18 da dt -X(x2 - 4) dt