Find a simple v or u function to establish the stability or instability, respectively, of the zero solution of the following equations. (v) x = sin y, ? =-2x-3y; (vi) x = x + e-v-1, ý x; (vii) x = ex-cos y, ? = y; (viii) x = sin (y +x), y =-sin (y-x); (ix) x=x3 (x) x = x + 4y, ? =-2x-5y; (xi) x=-x + 6y, ? = 4 x+y.