sqrt and norm doesn't return exact value

Hello guys I'm new to matlab and i ran into a problem where the code doesn't return the exact value in my case, i want to calculate the norm which is sqrt(a^2+b^2)

syms x y 

f(x,y)=((x-1)^2+10*(y-x^2)^2)

f(-1,2)

fx=gradient(f,x)

fy=gradient(f,y)

g=[fx(-1,2) fy(-1,2)]

normg=norm(g)

normg2=((g(1)^2+g(2)^2)^0.5)

normg3=sqrt(g(1)^2+g(2)^2)

I've tried doing it manually, and with norm and sqrt function but still giving me 1696^(1/2) which is correct but i want it to give me 41.18252

any thoughts? thank you

Answer 1

[Geoff Hayes]

Sam - see Perform Symbolic Compuations for some ideas on how to simplify the output of a symbolic expression. In your case, that would be

 simplify(normg2)
 simplify(normg3)

Try the above and see what happens!

[sam]

it is still the same answer :(

[Geoff Hayes]

Did you check the link on performing symbolic computations? Please post the results from each of

 >> normg3

and

 >> simplify(normg3)

What happens if you try

 >> eval(normg3)

Answer 2

[MA]

syms x y 
f=@(x,y)((x-1)^2+10*(y-x^2)^2);
vpa(f(-1,2))
fx=gradient(f,x);
fx=subs(fx,x,-1);
fx=subs(fx,y,2);
fy=gradient(f,y);
fy=subs(fy,x,-1);
fy=subs(fy,y,2);
g=[fx fy]
normg=norm(g)
normg2=((g(1)^2+g(2)^2)^0.5)
normg3=sqrt(g(1)^2+g(2)^2)

[sam]

nothing actually changed, still the same output

Answer 3

[Matt]

Sam,

I think this does what you're asking

double(normg2)
double(normg3)

Also the eval method that Geoff suggested would work as well, but casting from symbolic to double is a less ambiguous method.