10.设F1(x)，F2(x)分别为随机变量X1和X2的分布函数，且F(x)=aF1(x)一bF2(x)也是某一随机变量的分布函数，证明a—b=1．

<p><img src="/s/tiw/bjupload/2017-07-03/ac01d560-75ff-49d6-af89-7a6fcb5340b4.png" title="37.4.png">故<span>a</span><span>—b=</span><span>1．</span><br></p>
故a—b=1．