1) Note that 10=30-20, 50=30+20 and 70=90-20. Then think sin(A+B)=sinAcosB+sinBcosA.

2) Again this is just trig formula sin(A+B)=sinAcosB+sinBcosA and the one for cos as well.

3) 3A=108 2A=72. Note that 90+18=108 and 90-18=72 and then look at a graph of y=sin(x).

Let me know how you get on.