If the normals at a,b,c on the parabola l/r=1+cosμ be concurrent at the point (p,phi), then show that 2(phi)=a+b+c

Question

If the normals at \(\alpha, \beta, \gamma\) on the parabola \(\)\(l\)/r=1+cos\(\theta\) be concurrent at the point (\(\rho\),\(\phi\)​​​), then show that    2\(\phi\)=\(\alpha+\beta+\gamma\)