17.求 =a-(x)^2 与x轴所围成图形的面积 ()-|||-A. dfrac (1)(2)(a)^dfrac (1{2)} B. dfrac (4)(3)(a)^dfrac (3{2)} C. dfrac (4)(3)(a)^dfrac (1{2)} D. dfrac (1)(2)(a)^dfrac (3{2)}