O为等腰△ABC的底边AB的中点,以点O为圆心,AB为直径的半圆分别交AC,BC于点D,E.
某放射性元素质量为M,测得每分钟放出1.2×104个β粒子,21天后再测,发现每分钟放出1.5×103个β粒子,该放射性元素的半衰期是多少?
The old man lives _____ in a ______ house, but he doesn't feel_____.