Please help! Am trying to find a

syms a
eqn = 5000 + 6800 - 10876 * ((1+a).^2-1)./(a(1+a)).^2 * 1./(1+a).^2 == 0;
sola = solve(eqn, a)

This is the error

Error using sym/subsindex (line 814)
Invalid indexing or function definition. Indexing must follow MATLAB indexing. Function arguments must be symbolic variables, and function body must be sym expression.

Answer 1

[Star Strider]

You are missing a multiplication operator. MATLAB does not recognise implied multiplication.

eqn = 5000 + 6800 - 10876 * ((1+a).^2-1)./(a(1+a)).^2 * 1./(1+a).^2 == 0;
                                           ↑ ← INSERT ‘*’ HERE

With that addition:

syms a
eqn = 5000 + 6800 - 10876 * ((1+a).^2-1)./(a*(1+a)).^2 * 1./(1+a).^2 == 0;
sola = solve(eqn, a)

produces:

sola =
 root(z^5 + 4*z^4 + 6*z^3 + 4*z^2 + (231*z)/2950 - 2719/1475, z, 1)
 root(z^5 + 4*z^4 + 6*z^3 + 4*z^2 + (231*z)/2950 - 2719/1475, z, 2)
 root(z^5 + 4*z^4 + 6*z^3 + 4*z^2 + (231*z)/2950 - 2719/1475, z, 3)
 root(z^5 + 4*z^4 + 6*z^3 + 4*z^2 + (231*z)/2950 - 2719/1475, z, 4)
 root(z^5 + 4*z^4 + 6*z^3 + 4*z^2 + (231*z)/2950 - 2719/1475, z, 5)

However if you use vpasolve*:

sola = vpasolve(eqn, a)

the result is:

sola =
                                         0.4782949577645820345467328553868
 - 1.6314960838510317431154807318846 + 0.37168279788646925652089633009105i
 - 1.6314960838510317431154807318846 - 0.37168279788646925652089633009105i
  - 0.60765139503125927415788569580884 + 1.003620755351086792943582765314i
  - 0.60765139503125927415788569580884 - 1.003620755351086792943582765314i

which is somewhat more informative.