If a+b+c=11 and a2+b2+c2=81, then find ab+bc+ca.
Formula (a+b+c)2=a2+b2+c2+2(ab+bc+ca)Substituting a+b+c=11;a2+b2+c2=81112=81+2(ab+bc+ca)121−81=2(ab+bc+ca)ab+bc+ca=402=20
If ab+bc+ca=36 and a2+b2+c2=85 then find a+b+c.