1) (3d+5c)^2-(3d-5c)^2=9d^2+30cd+25c^2-9d^2+30cd-25c^2=60cd
2) p=2*(a+b)
p/2=a+b
a=p/2-b
9d^2+30dc+25c^2-9d^2+30dc-25c^2=60dc
a+b= p/2
a=p/2 - b
