填空题:设离散型随机变量X的分布函数为 <br /> <img src="http://www.weimei168.com/uploads/202208/161223642524331.png" width="214" height="91" /><br /> 且P{X=-1)=<img src="http://www.weimei168.com/uploads/202208/16122364324701.png" width="20" height="52" />，则a=_______.