1) (b² + 81)/(b - 12)(b + 12) + 6/(b + 12) =
(b² + 81 + 6b - 72)/(b - 12)(b + 12) = (b² + 6b + 9)/(b - 12)(b + 12) =
= (b + 3)² /(b - 12)(b + 12)
2) [(b + 3) / (b - 12)] : [ (b + 3)² /(b - 12)(b + 12) ] =
= [(b + 3)*(b - 12)(b + 12)] /[ (b - 12)(b + 3)²= (b + 12) / ( b+ 3)