Solution
Correct Answer: Option D
দেওয়া আছে,
ab + bc + ca = 31
a2 + b2 + c2 = 19
এখন,
(a + b + c)2 = a2 + b2 + c2 + 2ab + 2bc + 2ca
বা, (a + b + c)2 = a2 + b2 + c2 + 2(ab + bc + ca)
বা, (a + b + c)2 = 19 + (2 × 31)
বা, (a + b + c)2 = 19 + 62
বা, (a + b + c)2 = 81
বা, a + b + c = √81
∴ a + b + c = 9