trigo problem no.6

p181 q11 ex8b

question:

show that the equation acosx + bsinx +c = 0 has only one root in the range of 0 =< x < 360 if and only if a^2 + b^2 + c^2 .

though i know the discrimination D = 0 i don't know how to do the proving as it is not a quadratic equation.