Why subs symbolic fails here

How to make subs work for expression that are not identically simplified from the 'simlify' function.

The expressions bellow are not fully simplified and subs do not substitute.

clear all; close all; clc;
syms  a K L
syms m1 b positive;
c = (cos(b)^2 - 2*m1*cos(b)*a)/m1+(m1 - 2*m1^2*cos(b)*a)/(2*m1^2);
cf = subs((c), {cos(b)^2/m1 - 2*cos(b)*a,1/m1 - 2*cos(b)*a}, {K,L})

[John D'Errico]

Why would you possibly have expected it to succeed? For example, you have the expression:

cos(b)^2/m1 - 2*cos(b)*a

that you wish to replace with something. Yet, there is not even any term of that form. Yes, with algebraic simplicification, you can find a sub-expression like that. But it is not even an issue of simplification, but of factorization. Subs cannot work like that.

[Ole]

Because I would like to have simpler expresions.

Then perhaps the question is how to substitute such expressions.

Answer 1

[Walter Roberson]

cf = subs(2*expand(c), {cos(b)^2/m1 - 2*cos(b)*a,1/m1 - 2*cos(b)*a}, {K,L})/2

cf =
 
K + L/2