ta có (√a+√b)/2(√a-√b)  -(√a-√b)/2(√a+√b)-2b/(b-a)
      =(√a+√b)^2/2(a-b)-(√a-√b)^2/2(a-b)+4b/2(a-b)
      =(a+2√ab+b-a+2√ab-b+4b)/2(a-b)
      =4(√ab+b)/(√a-√b)(√a+√b)
      =4√b(√a+√b)/(√a-√b)(√a+√b)
      =4√b/(√ a-√ b)