MATLAB: Solve function not working

Question

Well here my code:

clear all
clear vars
syms x y 
y=1+(x).^(2)
x=-1
a=subs(y)
x=3
b=subs(y)
c=(b-a)./(4)
yy=diff(y)
disp(yy)
ans=solve(yy==c,x)
disp(ans)

And according to me 2x=2 and hence x=1 therefore a solution must exist but the output is Empty sym: 0-by-1

Answer 1

You have

x=3

so when you do

ans=solve(yy==c,x)

that is the same as

ans=solve(yy==c,3)

and 3 is never equal to 0, so the system is unsolvable.

I recommend that you avoid this problem in future by never assigning a value to a symbolic variable that has been used in an expression -- not unless you proceed to immediately overwrite the expression with the new variation on the variable. Use the three-parameter version of subs() instead.

x1=-1
a=subs(y,x,x1)
x2=3
b=subs(y,x,x2)

Now x is still a symbolic variable.