Explanation:
remember, the circumference of a circle is
2×pi×r
and that is for the full 360° of a circle.
how long will the arc length of 1° be ?
well,
2×pi×r / 360
and how long will the arc length of 78° be ?
well,
78 × 1° = 2×pi×r × 78/360
you see ? that is the whole trick ...
now we only need to enter the value for r (radius).
and that is GH = 9 units.
so, the length of the arc GJ is
2×pi×9 × 78/360 = 2×pi × 78/40 = pi × 78/20 =
= 12.25221135... ≈ 12.25 units