i am having difficulties figuring out this question on my assignment

Express f(theta)=3 cos(theta) - 4 sin(theta) in the form rcos(theta)+1

0<=r and 0<=(alpha)<=pi/2

and find the (i)Find the Maximum value of f(theta)

(ii)Find the minimum value of 1/8+f(theta)

and NO.2- Given that sum of angles ABC of a triangle is pi radians, show that sinA =sin(b+c)

part b : sinA+sinb+sinc=sin(a+b)+ sin(b+c)+sin(a+c)

i am having difficulties anwsering these two questions as i have not yet to learn them in class yet....the teacher has not taught us this yet but still gave it to us

{caribbean teachers at my school}

would really appreciate a work through with this one, as no body in my class knows how to do it^

Thank you very much