Question : If $(d+e+f)=14$, $(d^2+e^2+f^2)=96$, then find the value of $(de+ef+fd)$.
Option 1: 75
Option 2: 25
Option 3: 50
Option 4: 100
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Correct Answer: 50
Solution :
Given: $d^2+e^2+f^2=96$ and $d+e+f=14$
We know, $(d+e+f)^2=d^2+e^2+f^2+2(de+ef+fd)$
⇒ $14^2 = 96 + 2(de+ef+fd)$
⇒ $de+ef+fd=50$
Hence, the correct answer is 50.
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