Question

why convex mirror is used as side view of the vehicles and why

Answer 1

Convex mirror is used as rear view mirrow as it gives a clear diminished and an erect image of the traffic that is behind you.It covers a wide range of the traffic behind you and gives a very clear image . 
Just imagine the fact that you are using a concave mirror . Everything behind you will be enlarged and tilted from the original position and you cant see the actual traffic behind you . Even a plain mirror is not used as it does not cover a wide surface for the traffic. 
Hence, a convex mirror is used .
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Answer 2

Convex mirror is used as side view of vehicle because the image formed by the convex mirror is small and the people can easily
see what is behind his vehicle.
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