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A sharp knife is better than blunt knife because a sharp knife has less and pointed area whereas a blunt knife has broader area . We know that pressure increase when area decrease. That's why sharp knife is better than blunt knife.
a sharp knife has lesser area of contact with the surface which it tries to cut, 
since area of contact is less, with the same amount of force applied by you, you exert a greater pressure (as compared while using a blunt knife) on the surface which you want to cut. this makes the sharp knife preferable. 
also, a sharp knife will provide precision while cutting.