正负根号34，约等于正负5.830951895
设y=x^0.5,则y'=0.5x^-0.5,取x=36,dx=-2得dy=-1/6。所以34^0.5=y+dy=35/6=5.83
约等于±5.830951895
34的开方是开不尽的，约等于5.83这样
±5.8309518948453004708741528775456
