... chun b 1. 1 H phng trỡnh vi phõn cp mt nh ngha 1. 1 H n phng trỡnh vi phõn cp mt l h dx1 = f1 (t, x1 , x2 , , xn ) dt dx2 = f2 (t, x1 , x2 , , xn ) dt dx n = fn (t, x1 , ... vect (x1 , x2 , , xn ), f l vect (f1 , f2 , , fn ) thỡ h (1. 1) vit c di dng vect sau x = f (t, x) Ta xột trng hp c bit ca h (1. 1) cỏc v phi khụng ph thuc vo t, tc l dx1 = f1 (x1 , x2 ... x1 , x2 , , xn ), dt (1. 1) ú, t R l bin s c lp, x1 = x1 (t), x2 = x2 (t), , xn = xn (t) l cỏc hm n cn tỡm Cỏc hm fi vi i = 1, , n xỏc nh I Rn +1 H n hm kh vi x1 = (t), x2 = (t), ,...