ab +bc = 69 b(a+c) = 69 a/8 = b/12 = c/15 a = 8b/12 = 2b/3 c = 15b/12 = 5b/4 b(2b/3 + 5b/4) = 69 b( 8b/12+15b/12) = 69 b × 23b/12 = 69 23b^2 = 69×12 23b^2 = 828 b^2 = 828 : 23 b^2 = 36 b = 6 a = 2×6/3 = 4 c = 5×6/4 = 30/4 = 7,5 a = 4 b = 6 c = 7,5