Let f be differentiable for all x. If f(1)=−2 and f'(x)≥2 for all x∈(1,6], then which of the following cannot be the value of f(6)?
A
9
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B
10
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C
6
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D
7
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E
8
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Solution
The correct option is D 7 By Lagrange's Mean Value Theorem, there exists c∈(1,6) such that f′(c)=f(6)−f(1)6−1 f(6)+25≥2(∵f′(x)≥2for all x∈[1,6]) f(6)+2≥10 or f(6)≥8 ∴ 6 and 7 cannot be the value of f(6)