matlab Error using ==> solve is not a valid expression or equation.syms x l r n b kf1=('cos(pi/2-l/bn)=(r.^2+(x+sqrt(k.^2-r.^2)).^2-k.^2)/(2*r*(x+sqrt(k.^2-r.^2)))');[a]=solve(f1)Error using ==> solve' cos(pi/2-l/bn)=(r.^2+(x+sqrt(k.^2-r.^2)).^2-k.^2