Question

If a + b + c = 9 and ab + bc + ca = 26 find the value of a^2 + b^2 + c^2​

Answer 1

`(a+b+c)^2=a^2 + 2 a b + 2 a c + b^2 + 2 b c + c^2\\a^2+b^2+c^2=(a+b+c)^2-2ab-2ac-2bc\\a^2+b^2+c^2=(a+b+c)^2-2(ab+ac+bc)\\\\a^2+b^2+c^2=9^2-2\cdot26=81-52=29`