A:a、b都带正电
B:a、b都带负电
C:a带正电,b带负电
D:a带负电,b带正电
—Is Jack in the classroom now?
—No, he ________ football on the playground.
已知sinα=45,α∈(π2,π),cosβ=﹣513,β是第三象限角,求cos(α﹣β)