已知x+y=10,xy=5,求x2+y2的值.
将x+y=10两边平方得:(x+y)2=x2+2xy+y2=100,
将xy=5代入得:x2+y2=90.
一根阻值是1OΩ的导体,用拉丝机在温度不变的情况下将其均匀拉长到原来的2倍 , 则导体电阻变为______Ω。
.
Putting down he receiver, he__________his umbrella and made for the door.
A.put up
B.brought up
C.opened up
D.picked up