Explanation:
the whole 120° sector of the circle is the sum of the white isoceles triangle (120° top angle, 2 equal sides of 4) and the shaded segment.
so, to get the area of the shaded segment, we need to calculate the area of the sector and subtract the area of the triangle.
as the area of the full circle (360°) is
pi × r²,
the area of a sector with angle theta is
theta/360 × pi×r²
simply the theta/360 part of the whole circle.
so, in our case that means
120/360 × pi×4² = 1/3 × 16pi = 16pi/3
the area of the white triangle is a bit trickier.
in general it is
baseline × height / 2
for a right-angled triangle that means
leg1 × leg2 / 2
now, if we draw the height in the main triangle, this splits the main triangle into 2 equal right-angled triangles. theta gets split in half as well (120/2 = 60°).
and the area of one of them is then
(half of main baseline) × height / 2
and we get 2 of them, so the area the main triangle is
(half of main baseline) × height
how long are the height and half of the main baseline ?
we know from trigonometry that such a right-angled triangle with theta/2 as angle at the center of the circle makes
half of main baseline = sin(theta/2)×r
height = cos(theta/2)×r
remember, in any circle larger than r = 1 we need to multiply the trigonometric functions sine and cosine by the radius to get the actual lengths.
so, the area of the main triangle is
sin(theta/2)×r × cos(theta/2)×r =
= sin(theta/2)×cos(theta/2)×r² =
= sin(60)×cos(60)×4²
and therefore, the area of the shaded segment is
16pi/3 - sin(60)×cos(60)×4² =
= 16pi/3 - sin(60)×1/2 × 16 =
= 16pi/3 - sin(60)×8 = 9.826957589... ≈ 9.8 units²