两条平行直线img class=kfformula src=zzximg/20190809/1565322829875215.png data-latex=3x+4y-7=0/img class=kfformula src=zzximg/20190809/1565322829154016.png data-latex=3x+4y+8=0/之间的距离是( )
①
img class=kfformula src=zzximg/20190809/1565322829833725.png data-latex=\frac {1} {5} width=87 height=63/
②
img class=kfformula src=zzximg/20190809/1565322829406438.png data-latex=15/
③
img class=kfformula src=zzximg/20190809/1565322829721740.png data-latex=3\sqrt {2}/
④
img class=kfformula src=zzximg/20190809/1565322829886592.png data-latex=3/