The sun (diameter D) subtends an angle of θ radians at the pole of a concave mirror of focal length f. The diameter of the image of the sun formed by the mirror is

$fθ$

Since the sun is at very large distance, p = ∞

$⇒\frac{1}{∞}+\frac{1}{q}=\frac{1}{f}$

$⇒g=f$

If the diameter of the image be 

$\frac{d/2}{q}=α⇒d=(2α)q$

Putting $2α = θ$ and $q = f$

we obtain $D=fθ$