y = [根号下（cosx²）]'=[(cosx²)^(1/2)]'= (1/2)×(cosx²)^(-1/2)×(cosx²)'= (1/2)×(cosx²)^(-1/2)×(-sinx²)×(x²)'= (1/2)×(cosx²)^(-1/2)×(-sinx²)×...