Question 54588
You have to be patient with these.  Take as many steps as you need to to isolate your variables.
:
27. {{{r=ab/(a+b)}}} for a  
{{{r(a+b)=(a+b)*(ab)/(a+b)}}}
{{{r(a+b)=cross((a+b))*(ab)/cross((a+b))}}}
{{{r(a+b)=ab}}}
{{{ar+br=ab}}}
{{{-ar+ar+br=ab-ar}}}
{{{br=ab-ar}}}
{{{br=a(b-r)}}}
{{{br/(b-r)=a(b-r)/(b-r)}}}
{{{br/(b-r)=a*cross((b-r))/cross((b-r))}}}
{{{highlight(br/(b-r)=a)}}}
:
28.  {{{C=K(Rr/(R-r))}}}
{{{C(R-r)=K(Rr)(R-r)/(R-r)}}}
{{{C(R-r)=K(Rr)cross((R-r))/cross((R-r))}}}
{{{CR-Cr=KRr}}}
{{{-CR+CR-Cr=KRr-CR}}}
{{{-Cr=KRr-CR}}}
{{{-Cr=R(Kr-C)}}}
{{{-Cr/(Kr-C)=R(Kr-C)/(Kr-C)}}}
{{{-1(-Cr)/-1(Kr-C)=R*cross((Kr-C))/cross((Kr-C))}}}
{{{Cr/(-Kr+C)=R}}}
{{{highlight(Cr/(C-Kr)=R)}}}
Happy Calculaitng!!!