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

A20

B30

C40

D50

Answer:

D. 50

Read Explanation:

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


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