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x?? + ()fx ()g x x?? ( ) ( )f x g x , ???xy ( ) ( )f x x f xx?? ?? ()g x x??+ ()fx ( ) ( )g x x g xx?? ?? 因為 ()gx在點 x處可導(dǎo),所以它在點 x處連續(xù),于是當(dāng) 0??x 時, ( ) ( )g x x g x? ? ? , 從而0lim??x ???xy 0lim??x ( ) ( )f x x f xx?? ?? ()g x x??+ ()fx0lim??x ( ) ( )g x x g xx?? ?? 39。( ) ( ) ( ) 39。( )f x g x f x g x??, 法則 4 兩個函數(shù)的商的導(dǎo)數(shù),等于分子的導(dǎo)數(shù)與分母的積,減去分母的導(dǎo)數(shù)與分子的積,再除以分母的平方,即 39。2( ) 39。( ) ( ) ( ) 39。(