r=ab/a+b+c
r*(a+b+c)=ab
ra+rb+rc-ab=0
(ra-ab)=-rb-rc
a(r-b)=-r(b+c)
a=-r(b+c)/(r-b)
a=r(b+c)/(b-r)
a5= a1+4d
35=a1+4*6
35=a1+24
a1=35-24
a1= 11