Matlab：评估二阶导数

Question

在我取得函数f的第二个偏导数后，

second_div = diff(f,pz,2);

second_div现在应该是pz和u

的函数

我想在second_div评估(pz,u)。我该怎么做？

ATTEMPTS

second_div(2,3); <- failed because second_div is a 1*1 sym
vpa(subs(second_div,pz,u_m,2,3)); <- failed
vpa(subs(second_div,(pz,u_m),(2,3))); <- failed

感谢。

Answer 1

从mathworks.com/help/symbolic/subs.html开始，您可以替换{}中的变量列表：

syms f pz u
f = u*pz^3
second_div = diff(f,pz,2);
subs(second_div,{pz,u},[3,4])

按预期提供72。