I keep getting the message: Unbalanced or unexpected parenthesis or bracket. How would I fix it? I'm trying to solve for the maximum and minimum with a function and it's constraint.

syms x y lam
f = 2*x^4-x^2+3*y^2
g = x^2+y^2+3*y-9
fx = diff(f,x)
fy = diff(f,y)
gx = diff(g,x)
gy = diff(g,y)
[alam ax ay] = solve( fx-lam*gx, fy-lam*gy)
[bx by] = solve( gx,gy )
T = [ ax ay subs(g,{x y},{ax ay}) subs(f,{x y}, {ax ay })({bx by}) subs(g,{x y},{
bx by}) subs(f,{x y}{bx by})]
double(T)
double(T([1,2,3,4,7,8],[1,2,3,5]))

Answer 1

[Steven Lord]

T = [ ax ay subs(g,{x y},{ax ay}) subs(f,{x y}, {ax ay })({bx by}) ...
     subs(g,{x y},{bx by}) subs(f,{x y}{bx by})]

This part is not valid MATLAB syntax.

subs(f,{x y}, {ax ay })({bx by})

I suspect what you want it to make T a two row matrix where the first consists of ax and ay and the results of substituting those into g and f and the second consists of bx and by and the results of substituting those into g and f. If that's the case:

T = [ ax, ay, subs(g, {x y}, {ax ay}), subs(f, {x y}, {ax ay}); ...
      bx, by, subs(g, {x y}, {bx by}), subs(f, {x y}, {bx by})]

If that's not what you want explain in words, not code, what you expect / want that section of code to do. Once we know your goal we may be able to help you express that in code.

[Alejandro Avelar]

I think that the mistake I did was accidently was solving for fz and gz in a previous problem and so ended up with too many rows/ columns. I think that what you did is correct. I corrected my code and got:

f =

2*x^4 - x^2 + 3*y^2

g =

x^2 + y^2 + 3*y - 9

fx =

8*x^3 - 2*x

fy =

6*y

gx =

2*x

gy =

2*y + 3

alam =

0

0

0

ax =

0

-1/2

1/2

ay =

0

0

0

bx =

0

by =

-3/2

T =

[ 0, 0, -9, 0, 0, -3/2, -45/4, 27/4]

[ -1/2, 0, -35/4, -1/8, 0, 0, 0, 0]

[ 1/2, 0, -35/4, -1/8, 0, 0, 0, 0]

ans =

0 0 -9.0000 0 0 -1.5000 -11.2500 6.7500

-0.5000 0 -8.7500 -0.1250 0 0 0 0

0.5000 0 -8.7500 -0.1250 0 0 0 0

Index exceeds matrix dimensions.

Error in sym/subsref (line 841)

R_tilde = builtin('subsref',L_tilde,Idx);

Error in Untitled (line 13)

double(T([1,2,3,4,7,8],[1,2,3,5]))

Any idea on why the last part is like that with the error?

[Alejandro Avelar]

Essentialy, I want to make 3 colums with 4 rows. the 3 columns with x , y, and the max/min values . the 4 rows would be for fx,fy, gx, gy. I am stuck on the double T

[Steven Lord]

Your T matrix only has 3 rows and 8 columns, so asking for something in the 4th, 7th, or 8th row of T can't work. If you were to transpose T so it's an 8-by-3 those row indices would work, but you can't ask for something in the 5th column of a 3 column matrix.

Answer 2

[Matt J]

The error message tells you where to look: Line: 11 Column: 21

[Alejandro Avelar]

I understand that. I keep moving the brackets, replaciong them with parenthesis because I heard they work better for matlab but nothing is changing.

[Matt J]

You are missing a comma at line 11 column 21,

 subs(f,{x y},{bx by}
             ^ 
             |  
            here

[Alejandro Avelar]

I put comma where you said and also put a comma on line 10 between ay and =bx.

I got output of

f =

2*x^4 - x^2 + 3*y^2

g =

x^2 + y^2 + 3*y - 9

fx =

8*x^3 - 2*x

fy =

6*y

gx =

2*x

gy =

2*y + 3

alam =

0

0

0

ax =

0

-1/2

1/2

ay =

0

0

0

bx =

0

by =

-3/2

T =

[ 0, 0, -9, 0, 0, -3/2, -45/4, 27/4]

[ -1/2, 0, -35/4, -1/8, 0, 0, 0, 0]

[ 1/2, 0, -35/4, -1/8, 0, 0, 0, 0]

ans =

0 0 -9.0000 0 0 -1.5000 -11.2500 6.7500

-0.5000 0 -8.7500 -0.1250 0 0 0 0

0.5000 0 -8.7500 -0.1250 0 0 0 0

Index exceeds matrix dimensions.

Error in sym/subsref (line 841)

R_tilde = builtin('subsref',L_tilde,Idx);

Error in Untitled (line 13)

double(T([1,2,3,4,7,8],[1,2,3,5]))

Any idea on the error for the last part?

[Alejandro Avelar]

Essentialy, I want to make 3 colums with 4 rows. the 3 columns with x , y, and the max/min values . the 4 rows would be for fx,fy, gx, gy. I am stuck on the double T