试用虚位移原理求图示简支梁的挠曲线，并求解跨度中点处的挠度（忽略剪切变形的影响）。<br /> <img src="https://nimg.ppkao.com/2019-05/wangjing/2019050716103487251.jpg?sign=9b604b4a61ddd6af62fa3eba0ab5a354&t=62d682e5" />