Try this:
f1 =@(x) (1./(cosh(x).*cos(x)-1)).*(16.*(-(x.^3).*(cosh(x).*sin(x) + cos(x).*sinh(x))).*(-x.*(cosh(x).*sin(x)-cos(x).*sinh(x)))-4.*(-(x.^2).*sinh(x).*sin(x)).*(-(x.^2).*sinh(x).*sin(x)));
x=0.1:0.1:10;
plot(x,f1(x))
hold on
xlim([0 12])
grid on
title('Plot of Determinant against k - CC')
xlabel('k')
ylabel('Determinant')
hold off
