3D Trig Question Proving

[SHB92MX]

Hope you all can see this clearlybut im having trouble working some of these out.
If possible, could someone help explain seting out the working out for the second question?
Thanks

 

[m_a_t_h]

If you look at the question, we know that h=DF. and in the triangle ADF, sin(20)=DF/AD.
In the triangle AGD, cos(30)=AD/AG


Therefore:


sin(20) x cos(30) = (DF/AD) x (AD/AG)
sin(20) x cos(30) = DF/AG
AG = DF/sin(20) x cos(30)