Diketahui luas permukaan suatu kerucut 438,815 dm2 jika jari jarinya 6,5 dm tentukan luas selimut kerucut tersebut

Lp = phi.r(s+r)
438,815 = 3,14.6,5(s+6,5)
438,815 = 20,41(s+6,5)
438,815 = (20,41 × s)(20,41 × 6,5)
438,815 = 20,41 s.132,665
438,815-132,665 = 20,41s
306,15 = 20,41 s
306,15/20,41 = s
s =15 dm

Ls = phi.r.s
= 3,14.6,5.15
= 306,15 dm^2