f'(x) = 3ax² +2bx = x(3ax + 2b)= 0
x = 0æx = -2b/(3a)
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(ii) -2b/(3a) < 0, b >0
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(i) 0 < -2b/(3a), b > 0
æ¤æ¶f(0) = 0æf(-2b/(3a)) = 0
ä½f(0) = -2,æ以åè
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a < 0, b > 0, æ¤å¼å¯è½
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(ii) -2b/(3a) < 0, b < 0
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