若a,b,c是三角形的三边之长,则代数式a -2ac+c -b 的值( )
A:小于0
B:大于0
C:等于0
D:以上三种 情况均有可能
用所给词的适当形式填空
My watch doesn’t work. It needs to ____ .Who ____ away my pen? I can’t find it.Lucy said she____ the primary school the next month. It’s correct____ strangers come into your house. It’s ____ to join a charity walk than to play computer games.
①∃x0∈R,ln(x02+1)<0;
②∀x>2,x2>2x;
③∀α,β∈R,sin(α﹣β)=sin α﹣sin β;
④若q是¬p成立的必要不充分条件,则¬q是p成立的充分不必要条件.
其中真命题的个数为( )
温度为0℃时,下列现象正确的是 ( )