As shown in the figure, in the trapezoidal ABCD, ad ‖ BC, ∠ B = 90 °, ab = 4cm, ad = 18cm, BC = 21cm, point P starts from point a, moves 2cm / s along edge ad to point D, point Q starts from point C, moves 6cm / s along edge CB to point B, P and Q start at the same time, if one point moves to the end point, the other point stops=______ Cm; 2______ Second, PQ = CD

As shown in the figure, in the trapezoidal ABCD, ad ‖ BC, ∠ B = 90 °, ab = 4cm, ad = 18cm, BC = 21cm, point P starts from point a, moves 2cm / s along edge ad to point D, point Q starts from point C, moves 6cm / s along edge CB to point B, P and Q start at the same time, if one point moves to the end point, the other point stops=______ Cm; 2______ Second, PQ = CD

① Let DH ⊥ BC be h, then the quadrilateral abhd is a rectangle. DH = AB = 4, BH = ad = 18. Ch = BC-AD = 21-18 = 3. According to the Pythagorean theorem, CD = 5 (CM). ② let PQ = CD after T seconds. According to the meaning of the title, AP = 2T, CQ = 6T. As shown in Figure 1, the quadrilateral PQCD is a parallelogram, then PD = CQ, that is, 18-2t = 6T, t = 2.25 (s); as shown in Figure 2, the quadrilateral PQCD is an isosceles trapezoid 6T - (18-2t) = 6, 8t = 24, t = 3 (SEC)