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(i)  The rays from sun after reflection from the mirror, shall converge at focus. Holding  a sheet of paper at the focus, may cause it to burn if the reflected rays are very strong but if a sheet of paper is held very close to the mirror, it will lie in between the pole and the focus and hence shall only warm up.

(ii) A convex mirror forms a virtual image of the sun. In a real image, the refracted rays pass though the image point. In a virtual image, the rays only appear to pass though the image point, but they actually do not. So if you want to concentrate that sunlight on a point, use a concave mirror and real image.
(iii) Radius of curvature is twice of focal length
R=2f