合并同类项:6x-10x2+12x2-15x=______.
原式=(6x-15x)+(12x2-10x2)=2x2-9x.
故答案为:2x2-9x.
如果指数函数f(x)=(a-1)x是R上的减函数,则a的取值范围是( )。
I was late for class yesterday ________ there was something wrong with my bike.[ ]
A.when
B.that
C.until
D.because