已知直角坐标平面内两点A(2,-1)和B(-1,3),那么A、B两点间的距离等于______.
A、B两点间的距离等于(2+1)2+(-1-3)2=25=5.
故答案为5.
蝴蝶的翅膀每秒钟振动五、六次,它所发出的声调低于20Hz,为______波,而海豚能能发出振动频率高于20000Hz的声音,为______波.上述两种频率的声音,人耳______听见(“都能”或“都不能”).
It ____how long the shock that explosions hit London transport system will stay in people's hearts.
A.abandons
B.considers
C.matters.
D.minds