I often help mother__________ housework after school.
A:do
B:doing
C:to doing
D:does
用所学化学知识判断.下列说法正确的是( )
pain fit injury breathe anxious rare diet symptom
已知曲线y=13x3+43,
(1)求f′(5)的值;
(2)求曲线在点P(2,4)处的切线方程.
已知|a|=6,|b|=3,a·b =-12,则向量a在向量b方向上的投影是( )