Matlab：评估二阶导数

Question

在我对函数f 进行二阶偏导之后，

second_div = diff(f,pz,2);

second_div 现在应该是pz 和u 的函数

我想在给(pz,u) 时评估second_div。我该怎么做？

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尝试

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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

谢谢。

Answer 1

从mathworks.com/help/symbolic/subs.html 看到，您替换了{} 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])

按预期给出72。