Answer-If a2 + b2 + c2 = 21, (a + b + 7) = 21, then find the value of (ab + bc + ca).

We know, a b c2 a2 b2 c2 2ab bc ca Or, 212 21 2ab bc ca Or, 441 21 2ab bc ca Or, ab bc ca 441 21 2 210