Factorization With Symbolic Terms

Hello! Does any one knows how can I group a specific term in a long equation?

For example, imagine i have the equation , which is equivalent to .

If I use the factor function, I obtain:

syms x
eq = x^2 + 6*x + 9
factor(eq,x)

ans = [ x + 3, x + 3]

However, I want to group those terms inside the eq itself, so I can keep manipulating the symbolic expression (eq = (x+3)^2). I am asking these because I am solving a Lagrange equation and I need to group the terms in the forma (x + y) and (x - y). Once they are grouped, I want to make a variable change: subs(eq,(x+y),z) and subs(eq,(x-y),w).

Thanks a lot!

[madhan ravi]

eq = x^2 + 6*x + 9 -> should be 9 not 3

Answer 1

[Walter Roberson]

simplify() can handle some cases. However it is not sophisticated so while it might find x^2+6*x+9 it will certainly not find x^2+6*x+11 = (x+3)^2+2

subs will also not dig out opportunities for substitution . subs(5*xx+5*yy,xx+yy,zz) will not notice the opportunity as it looks for exact node matches.

So generally you would proceed like

simplify(subs(TheExpression, y, z-x))

[Giulio Cesare Mastrocinque Santo]

Thanks a lot for your help!

Answer 2

[madhan ravi]

AFAIK - I don't think there is a possibility to do that in symbolic math toolbox

After all the struggles: see isequaln()

eq=x^2 + 6*x + 9
s=factor(eq)
 
if isequaln(s(1),s(2)) %assuming only two factors 
    result=s(1)*s(2);
else 
    result = s;
end 

[Giulio Cesare Mastrocinque Santo]

That works too! Thanks a lot for your answer!

[madhan ravi]

Anytime :) , you can vote for the answer if it was helpful.