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5. Continuity and Differentiation
normal
Suppose that $f$ is differentiable for all $x$ and that $f '(x) \le 2$ for all x. If $f (1) = 2$ and $f (4) = 8$ then $f (2)$ has the value equal to
A
$3$
B
$4$
C
$6$
D
$8$
Solution
Using $LMVT$ for $f$ in $[1, 2]$
$\forall c \in (1, 2)$ $\frac{{f(2) – f(1)}}{{2 – 1}}= f ' (c) \le 2$
$f (2) – f (1) \le 2==>f (2) \le 4$$….(1)$
again using $LMVT$ in $[2, 4]$
$\forall d \in (2, 4)\,\,\frac{{f(4) – f(2)}}{{4 – 2}}= f ' (d) \le 2 $
$f (4) – f (2) \le 4$
$8 – f (2) \le 4$
$4 \le f (2)$==>$f (2) \ge 4$.$…(2)$
from $(1)$ and $(2)$
$ f (2) = 4$
Standard 12
Mathematics