2)过S点做平行于主光轴的入射光线,经过凸透镜折射后要过S′,折射光线与凸透镜右侧的主光轴的交点就是凸透镜的焦点F位置,因为平行于主光轴的光线经过凸透镜折射后过焦点.
同理可以做出凸透镜左侧的焦点F.
答案如下图所示:
My father is interested in ____ (watch) films about history.
① ____ A. sweater B. shirt C. bus
② ____ A. football B. basketball C. soup
③ ____ A. fish B. wolf C. van
87×24=24×87运用了乘法结合律。