MonotonicityHard

Question

If f(x) = x2 + kx + 1 is monotonic increasing in [1 , 2] then the minimum values of k is-

Options

A.4
B.-4
C.2
D.-2

Solution

f(x) = x2 + kx + 1
f′(x) = (x2 + kx + 1) > 0 = 2x + k > 0
k > - 2x        x ∈ [1, 2]    k > -2

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