MATLAB: How to plot a linear approximation next to a function

approximationcurvefunctionslineartangent

Hey,
I have a function
f=@(x)0.5*(x-2).^2-2*cos(2*x)-1.5;
And I want next to the function f(x) to plot:
f(-1)+f'(-1)(x-(-1))
where f'(x)=x+4sin(2x)-2
How do I go about this?
I tried making a function file
function y=linjar(a)
a=-1;
f(a)=0.5*(a-2).^2-2*cos(2*a)-1.5
dy(a)=a+4*sin(2*a)-2;
y=f(a)+dy(a)*(x-a);
and then calling the function out on another script with
f=@(x)0.5*(x-2).^2-2*cos(2*x)-1.5;
x=linspace(-3,7);
plot(x,f(x))
axis([-3 7 -5 10]), grid on
hold on
plot(a,linjar(a,-1))
and I get the message: Error using linjar Too many input arguments.
I'm new to Matlab so ANY ideas where to start are welcomed.

Best Answer

As you wrote your ‘linjar’ function originally, it only takes one argument. In the line that’s throwing the error, you gave it two.
It would seem that what you want is actually:
function y=linjar(a,x)
f = @(a) 0.5*(a-2).^2-2*cos(2*a)-1.5;
dy = @(a) a+4*sin(2*a)-2;
y=f(a)+dy(a)*(x-a);
creating anonymous functions where you seem to want them, adding ‘x’ as an argument (because it will throw an error if ‘x’ is not defined somewhere), and eliminating the ‘a=-1;’ line because that would override the value of ‘a’ you provided as an argument.
I don’t understand the (-1) references. If you mean the value (-1), enter it as an argument. If you mean the inverse of your function, that requires a bit more code.
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