Question

if a+b+c=9 and ab+bc+ca=15. find
${a}^{2} + {b}^{2} + {c}^{2}$

Answer 1

(a+b+c)^2 = a^2 +b^2 + c^2 +2(ab+bc+ca)

9^2=a^2 +b^2 +c^2 + {2*15}

81-30 = a^2+b^2 +c^2

51=a^2 +b^2 +c^2