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5. Continuity and Differentiation
medium
Let $f (1) = - 2$ and $f ' (x) \ge 4.2$ for $1 \le x \le 6$. The smallest possible value of $f (6)$, is
A
$9$
B
$12$
C
$15$
D
$19$
Solution
Using $LMVT$ $\forall$ some $c \in (1, 6)$ s.t.
$f ' (c) =\frac{{f(6) – f(1)}}{5} =\frac{{f(6) + 2}}{5} \ge 4.2$ $f (6) + 2 \ge 21$
$f (6) \ge 19$
Standard 12
Mathematics
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