【正确答案】因为X与Y独立同分布，故
   P{a＜min(X，Y)≤b}=P{min(X，Y)≤b}-P{min(X，Y)≤a}
   =1-P{min(X，Y)＞b}-[1-P{min(X，Y)＞a}]
   =P{min(X，Y)＞a}-P{min(X，Y)＞b}
   =P(X＞a，Y＞a)-P(X＞b，Y＞b)
   =P(X＞a)P(Y＞a)-P(X＞b)P(Y＞b)
   =[P(X＞a)]²-[P(X＞b)]²