I want to fix the black area in the mirror. I used Glossy BSDF to make it and I'm using cycle render. And I want to know how to hide the cyan planes from the scene and keep their effect.
Starting with a scene like this:
Select the emitter objects and disable the ray visibility for camera and glossy.
As to why the round shape on your mirror.
If your mirror is rectangular you'll be gaining anything by adding a subsurf modifier to it. Subsurf modifier's default behavior is to round out geometry by interpolation:
If you must have the subsurf modifier change the interpolation from Catmull-Clark to Simple.