As shown in the figure, in the trapezoidal ABCD, ad ‖ BC, ∠ B = 90 °, ab = 14cm, ad = 18cm, BC = 21cm, point P starts from point a, moves at 1cm / s along side ad to point D, and point Q starts from point C, moves at 9cm / s along side CB to point B. If one point moves to the end point, the other point stops. If P and Q start at the same time, can a quadrilateral PQCD become an isosceles trapezoid? If so, how many seconds? If not, please give reasons

As shown in the figure, in the trapezoidal ABCD, ad ‖ BC, ∠ B = 90 °, ab = 14cm, ad = 18cm, BC = 21cm, point P starts from point a, moves at 1cm / s along side ad to point D, and point Q starts from point C, moves at 9cm / s along side CB to point B. If one point moves to the end point, the other point stops. If P and Q start at the same time, can a quadrilateral PQCD become an isosceles trapezoid? If so, how many seconds? If not, please give reasons

Let PQCD be an isosceles trapezoid, after T seconds, PD ‖ CQ, PQ = CD in the trapezoid PQCD; through P and D, make the vertical lines of BC, intersect BC at e, F, ∵ AB = 14cm, ad = 18cm, BC = 21cm, ∠ B = 90 °, PE = DF = AB = 14, ∵ CF = BC-AD = 21-18 = 3, ∵ after T seconds, AP = t, CQ = 9t