下列各式从左边到右边的变形是因式分解的为( )
A:(a+1)(a﹣1)=a2﹣1
B:﹣18x4y3=﹣6x2y2•3x2y
C:x2+2x+1=x(x+2x)+1
D:a2﹣6a+9=(a﹣3)2
— Why did she ______ your invitation?
— Because she had an important meeting to attend that day.
唐朝时,用“飞流直下三千尺,疑是银河落九天”的诗句勾画庐山瀑布壮丽图景的诗人是被人们称为 “诗仙”的( )
观察图《三国同盟和三国协约》,请指出哪一组数字代表的国家是同盟国集团?( )
若a+b=6,ab=4,则a2+b2=____