直线<img src="20190809/1565320469784241.png" title="1565320469784241.png" alt="图片33.png"/>的斜率为( )
①
<img class="kfformula" src="20190809/1565320507335017.png" data-latex="\frac {1} {2}"/>
②
<img class="kfformula" src="20190809/1565320507148737.png" data-latex="-\frac {1} {2}"/>
③
<img class="kfformula" src="20190809/1565320507128594.png" data-latex="2"/>
④
<img class="kfformula" src="20190809/1565320507871509.png" data-latex="-2"/>