For some reason, surprisingly few know this extremely simple formula, please share it.
I have actually seen in a carpet store an employee unrolling the carpet to measure the length!!
[The formula is simple to prove too… r=kφ, L=∫r dφ=∫kφ dφ=½k(φ2²-φ1²)=½k(φ2+φ1)×(φ2-φ1)=½(r2+r1) ×n2π=(r2+r1) × nπ , q.e.d.]