除法求導法則y=f(x)/g(x)y' = (f ' (x)g(x) - f(x)g ' (x))/g(x)^2（為看清楚哪個帶導數(shù)中間空開了）所以y=ax+b/cx+dy'=(ax+b)'(cx+d)-(ax+b)(cx+d)' / (cx+d)^2=a(cx+d)-c(ax+b) / (cx+d)^2=(ad-bc) / (cx+d)^2...（ax+b）'和(cx+d)'為什么是a和c（ax+b）的導數(shù)不就是a嗎？ax+b求導,ax導數(shù)是系數(shù)a,常數(shù)求導是0a+0=a