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While **force** is vector. So, we should use vector addition not "direct addition". 

From vector addition formula

$$C=\sqrt{A^2+B^2+2ABcos\alpha}$$

For force,

$$F=Eq$$
$$F=mg$$
$$F=\sqrt{(Eq)^2+(mg)^2+2Eqmgcos\alpha}$$

$$f=\frac{1}{2\pi} \sqrt{\frac{\sqrt{(Eq)^2+(mg)^2+2Eqmgcos\alpha}}{ml}}$$

Then, problem solved.