若（ $\overrightarrow{a}$ +3 $\overrightarrow{b}$ ）⊥（7 $\overrightarrow{a}$ ﹣5 $\overrightarrow{b}$ ），且（ $\overrightarrow{a}$ ﹣4 $\overrightarrow{b}$ ）⊥（7 $\overrightarrow{a}$ ﹣5 $\overrightarrow{b}$ ），则 $\overrightarrow{a}$ 与 $\overrightarrow{b}$ 的夹角大小为____．

在探究凸透镜成像的实验中，完成下面各题：

甲               乙

空间存在如图所示的匀强磁场，矩形闭合线圈平面跟磁感线方向平行。下列情况中线圈有感应电流的是（   ）

1开发氢能源；②禁止使用煤等燃料；③禁止乱砍乱伐；

④减少煤等燃料的使用；⑤利用水力发电；⑥种植花草树木．

    This is a s____  of twin brothers, Justin and Paul. Their friend Susan calls them for h____ . Justin and Paul drive to Hill Building q____ . Three man in police uniform p____  them into a green van. They are not p____ . They are the robbers. The twin brothers aren't a____ . Paul opens the back door of the van w____  a knife. They take different r____  to the police station. They meet the robbers a____  in the police station. They are s____ . The policemen stop the van because the back door of the van is open.