Let the points be A=(a‌cos‌θ,asin‌θ) and ‌B=(a‌cos‌φ,asin‌φ) ‌∴‌‌AB=√(a‌cos‌θ−a‌cos‌φ)2+(asin‌θ−asin‌φ)2 ‌=√a2cos2θ+a2cos2φ−2a2‌cos‌θ‌cos‌φ ‌+a2sin‌2θ+a2sin‌2φ−2a2sin‌θsin‌φ ‌=√2a2−2a2(cos‌θ‌cos‌φ+sin‌θsin‌φ) ‌=√2a(√1−cos(θ−φ) ‌⇒‌‌2a=√2a√2sin‌(‌