主要内容:
本文通过幂函数复合函数求导法、取对数求导法等,介绍函数y=(32x^3+11x+7)^4的一阶导数和二阶导数计算的主要步骤。
幂函数导数法:
此时看成是幂函数的复合函数,用链式求导,即:
∵y=(32x^3+11x+7)^4,即y=u^4,u=32x^3+11x+7,
∴y'=4*u^3*u’x=4*u^3*(96x^2+11),
即:y'=4(96x^2+11)(32x^3+11x+7)^3。
取对数求导法:
y=(32x^3+11x+7)^4,两边取对数得:
lny=ln(32x^3+11x+7)^4=4ln(32x^3+11x+7),
两边同时对x求导,得:
y'/y=4*(32x^3+11x+7)'/(32x^3+11x+7)
y'/y=4*(96x^2+11)/(32x^3+11x+7),进一步变形有:
y'=4(32x^3+11x+7)^4*(96x^2+11)/(32x^3+11x+7)
=4(32x^3+11x+7)^3*(96x^2+11)。
二阶导数计算:
使用函数乘积求法:
因为y'=4(96x^2+11)(32x^3+11x+7)^3,再次对x求导,所以:
y''=4[192x^1*(32x^3+11x+7)^3+(96x^2+11)*3*(32x^3+11x+7)^2*(96x^2+11)]
= 4[192x^1*(32x^3+11x+7)^3+(96x^2+11)^2*3*(32x^3+11x+7)^1]
= 4(32x^3+11x+7)^2*[192x^1*(32x^3+11x+7)^1+(96x^2+11)^2*3]
取对数求法:
对y'=4(96x^2+11)(32x^3+11x+7)^3取对数有:
ln y'=ln[4(96x^2+11)(32x^3+11x+7)^3]
=ln4+ln(96x^2+11)+3*ln(32x^3+11x+7)
两边同时对x再次求导,则:
y''/y’=192x^1/(96x^2+11)+3*(96x^2+11)/(32x^3+11x+7),进一步化简为:
y''= 4(96x^2+11)(32x^3+11x+7)^3*[192x^1/(96x^2+11)+3*(96x^2+11)/(32x^3+11x+7)]