a + b + c = 5 এবং a² + b² + c² = 9 হয়, ab + bc + ca = কত?

দেয়া আছে,
a + b + c = 5
a² + b² + c² = 9

আমরা জানি,
(a + b + c)² = a² + b² + c² + 2(ab + bc + ca)
5² = 9 + 2(ab + bc + ca)
25 = 9 + 2(ab + bc + ca)
25 - 9 = 2(ab + bc + ca)
16 = 2(ab + bc + ca)
ab + bc + ca = 16/2
ab + bc + ca = 8