已知a1=(1 3 2 0)^t a2=(7 0 14 3)^t a3=(2 -1 0 1)^t且(a1 a2 a3 a4 a5)经初等变换后得 (1 0 0 2 -1)(0 1 0 -1 1)(0 0 1 3 0)(0 0 0 0 0)求a4和a5