Question

Let a+b+c= 13, a² +b² + c² = 69. Then ab+bc+ca=...........?

• A. 20
• B. 30
• C. 40
• D. 50

Answer 1

Answer:

D. 50

Given (a+b+c) = 13, a² + b² + c² = 69 We have, (a+b+c)² = a² +b²+c²+2 (ab+bc+ac) 13² = 69 +2 (ab+bc+ca) (ab+bc+ca) = (13² - 69)/2 = (169 - 69)/2 = 100/ 2 = 50