几道数学题(初等算术)就能简单！

=3-1/3-1/9

=3-(3/9+1/9)

=23/9

2、15/16+(7/16-1/4)÷1/2

=15/16+3/16÷1/2

=15/16+6/16

=21/16

3、1/5÷[(2/3+1/5)*1/13]

=1/5÷[13/15*1/13]

=1/5÷1/15

=3