已知x,y > 0满足1/(2+x)+1/(2+y) = 1/4,求xy的最小值?换元以后会比较明显.设a = 1/(2+x),b = 1/(2+y),有a+b = 1/4,a,b > 0.于是x = 1/a-2 = 4(a+b)/a-2 = 4b/a+2.而y = 1/b-2 = 4(a+b)/b-2 = 4a/b+2.由Cauchy不等式,...