The fzero requires initial guess. Pick the initial value that is closest to the root:
fzero(@(x) x - 4*sin(x), pi)
and it will return another solution.
Can also try this Taylor series expansion method to "guess" the initial values:
syms x
fun = x - 4*sin(x);
T9 = taylor(fun, x, 'Order', 9)
fplot([fun T9])
grid on
xlabel('x')
ylabel('f(x)')
legend('x - 4*sin(x)', 'Taylor9', 'location', 'northwest')
p = sym2poly(T9);
g = roots(p);
g(imag(g) ~= 0) = [] % initial guesses of the approximated roots
r1 = fzero(@(x) x - 4*sin(x), g(1))
r2 = fzero(@(x) x - 4*sin(x), g(2))
r3 = fzero(@(x) x - 4*sin(x), g(3))
T9 = x^7/1260 - x^5/30 + (2*x^3)/3 - 3*x
g =
0
-2.4661
2.4661
r1 = 0
r2 = -2.4746
r3 = 2.4746
