Q5 of 82 Page 4

If x = 3 is one root of the quadratic equation x2 – 2kx – 6 = 0, then find the value of k. (CBSE 2018)

Given: x = 3 is a root of the equation f(x) = x2 – 2kx – 6 = 0
We know that if x = a is a root of a polynomial f(x), then f(a) = 0.

So, we can say that f(3) = 0


f(3) = 32 – 2k(3) – 6 = 0


⇒ 9 – 6k – 6 = 0
⇒ 3 – 6k = 0


⇒ 6k = 3


⇒ k = 1/2

Hence, k = 1/2

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