(1), (12 points) Let f(x, y)= if (x,y)≠(0,0) and f(0,0) = 0. Let = (a,b) be an unit vector. Find the directional derivative Daf(0,0). Is f differentiable at (0,0)? Give your reasons.
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if (x,y)≠(0,0) and f(0,0) = 0. Let 
= (a,b) be an unit vector. Find the directional derivative Daf(0,0). Is f differentiable at (0,0)? Give your reasons.