17 圖示靜不定梁受集中荷重 P 作用,A 點固端彎矩 M A = x PL (↻為正),B 點反力 RB = yP (↑為正),則(x, y)=?(A)(-1/2 , 1)(B)(-1/4 , 5/4)(C)(1 , 5/2)(D)(1/4 , 7/4)