如图，AC是电线杆AB的一根拉线，测得BC=6米，∠ACB=52°，则拉线AC的长为(          )

如图，一固定的竖直汽缸由一大一小两个同轴圆筒组成，两圆筒中各有一个活塞，已知大活塞的质量为m₁=2.50kg，横截面积为s₁=80.0cm²，小活塞的质量为m₂=1.50kg，横截面积为s₂=40.0cm²，两活塞用刚性轻杆连接，间距保持为l=40.0cm，汽缸外大气的压强为p=1.00×10⁵Pa，温度为T=303K，初始时大活塞与大圆筒底部相距 $\frac{1}{2}$ ，两活塞间封闭气体的温度为T₁=495K，现汽缸内气体温度缓慢下降，活塞缓慢下移，忽略两活塞与汽缸壁之间的摩擦，重力加速度大小g取10m/s²，求：

    Robert and Henry were going home from school, when, on turning a corner, Robert1out, "A fight! Let us go and see!" "No," said Henry; "Let us go quietly home and avoid this quarrel. We have2to do with it, and may get into trouble."

    "You are not a brave boy, and afraid to go, "said Robert, and off he ran. Henry went straight home, and in the afternoon went to school, as usual,3 Robert had told all the boys that Henry was a coward, and they laughed at him a great deal.

    A few days4, Robert was bathing with some schoolmates and 5his depth. He struggled and screamed for help, but all in vain. The boys 6 had called Henry a coward, got away from the water as fast as they could, but they did not even try to help him. Robert was fast 7, when Henry threw off his clothes, and sprang into the water. By great effort, and8much danger to himself, he brought Robert to the shore, and thus saved his life.

    Robert and his schoolmates were9at having called Henry a coward. They admitted that he had more10than any of them.

    Never be afraid to do good, but always fear to do evil.