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 My name is Dora.I’m American and I’m thirteen.I’m a student in No.7 Middle School.I like music very much.I can play two instruments— the guitar and the piano.I play them very well and I’m on the school art club.

Jeff and Michael are my brothers.They are students,too .But we aren’t in the same school.They like sports.Jeff plays basketball and soccer           very well.Michael swims and plays ping-pong very  well.I am not good at sports.They help me with sports after school.

We live in London with our parents now.My mother is an English teacher and my father is a musician.My mother swims well.My father can play the violin.

求证：$\sqrt{2}$+$\sqrt{3}$＞$\sqrt{5}$（　　）

如图所示是我国自主生产的某型号的水陆两用坦克，它的总质量22t，配备发动机的额定功率420kW．水上最大速度14km/h，陆地最大速度54km/h．（g=10N/kg）

（1）该水陆两用坦克漂浮在水面时，排开水的体积多大？（ρ_水=1.0×10³kg/m³）

（2）该水陆两用坦克停在水平地面上时，履带着地总面积为4m²，坦克对地面的压强是多大？

（3）该水陆两用坦克在陆地上以最大速度匀速行驶过程中，受到的牵引力多大？

世界上唯一地跨东西两个半球和两个大洲的国家是（  ）