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lim(h→0) [ √(9+h) - 3 ] / h
= lim(h→0) [ √(9+h) - 3 ] [ √(9+h) + 3 ] / h [ √(9+h) + 3 ]
= lim(h→0) [ (9+h) - 9 ] / h [ √(9+h) + 3 ]
= lim(h→0) 1 / [ √(9+h) + 3 ]
= 1/6