also, under the edit menu>preferences, you can set the preferred (default) radius, so if you need to draw several with the same radius... that can be your default. I usually and up doing the alt-click method also, so I can enter exact height & width parameters...
The prefs. method is handy when I'm laying out a banner or something & I'm working visually instead of mathmatically... I may not know how big my reverse panel needs to be to look right... but I know I want a 2" radius.
I wouldn't call that a radius. All your corners are sharp. If you mean the upper center section, you want to weld a segment of a circle.
draw the circle, draw the overlapping triangle, then draw a square around the top portion of the circle that you want to keep.
Open the "pathfinder" pallette, and with the square & circle selected, choose intersect. Now you will have your segment of a circle. This should just overlap the triangle by enough of a hairling that it's touching & then select both the triangle & the semicircle.. in the pathfinder, choose "unite"
Also, please upload .jpg images. If I was at my office computer, I would refuse to download a file onto my computer just to help someone, & I know other people feel the same.
Why? ...because it's a really really bad idea & nobody should ever download a file from an unknown source if they have anything of value on their computer. Yeah, that is an extreme position, & I don't mean to imply you or anyone else would send a malicious file... but the nature of malicious files is that they get spread unwittingly all the time, & it's just as simple to upload a .jpg