(2016•镇江)分解因式:x2﹣9=____
I'm afraid I'll spend _____ two hours _______on the problem.
____(.)
写出下列物质的结构式
(1)CH4 ____ (2)H2O ____ ,(3)Cl2 ____
(4)N2 ____ (5)CO2 ____ (6)NH3 ____