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
