一个n边形的每一个外角都等于72°,则n =____ ,它的内角和是____°.
The first question is as as the second one. I can answer them easily.
频率为6×1014 Hz的激光从S1和S2投射到屏上,若屏上的点P到S1与到S2的路程差为3×10-6 m,那么点P处是亮条纹还是暗条纹?设O为到S1和S2的路程相等的点,则PO间有几条暗纹、几条亮纹?(不含O、P两点处)
图片能反应一个时期的政治和军事。下列图片反映了一个主题,最恰当的是( )