The solution for problem #536 begins by having us use the formula: µ' = µ(sin(Φ/2)) ÷ (Φ+ sinΦ)
They help us by making sure we know to convert the angle of 80° to radians, 80° = 1.4rad
But when I put that into the equation, I don't get the same answer the book gets (book has 0.864, I get...