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If i try to write derivative of f in the below code(i pointed here), it doesn't work. why?

3 次查看（过去 30 天）
Shaik Merkatur Hakim Marjuban
评论： Birdman 2020-3-30
clear on
clc
syms x
x1=2.5;
f=@(x) x*sqrt(x)+2*x^3+1.5;
df=@(x)(3/2)*sqrt(x)+6*x^2; %[here]
f1= f(x1)+df(x1)*(x-x1);
figure(1)
fplot(f,[0 5])
hold on
fplot(f1,[0 5])
grid on

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回答（1 个）

Birdman
Birdman 2020-3-30
You do not have to use Symbolic approach this time. Try the following code:
hold off;
x1=2.5;
f=@(x) x.*sqrt(x)+2*x.^3+1.5; %f(x)
Df=@(x) 6.*x.^2+1.5.*sqrt(x); %derivative of f(x)
f1=@(x) f(x1)+Df(x1)*(x-x1);
fplot(f,[0 5]);hold on;fplot(f1,[0 5]);grid on;
  2 个评论
Shaik Merkatur Hakim Marjuban
Shaik Merkatur Hakim Marjuban 2020-3-30
clear on
clc
syms x
x1=2.5;
f=@(x) x*sqrt(x)+2*x^3+1.5;
df=@(x)diff(f,x); %[here]
f1= f(x1)+df(x1)*(x-x1);
figure(1)
fplot(f,[0 5])
hold on
fplot(f1,[0 5])
grid on
Why didn’this work?
Birdman
Birdman 2020-3-30
Firstly, get rid of the
syms x
line because after that line, you use numerical evaluation to get the result. Secondly, you have to define f1 as a function but instead, you define it as if it was a constant value. That is why it is not working. So, use my code to get the result.

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