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| PAGE: 174 | SET: Exercises | PROBLEM: 13 |

(a)
Area of the circle is
, Where
is a radius.
(i)
Average rate of change if radius
changes from
to
:
Average rate of change of
with respect to
is defined as
on the interval
.
Here
,
and
.
Average rate of change in area is
.

(ii)
Average rate of change if radius
changes from
to
:
Average rate of change in area is

(iii)
Average rate of change if radius
changes from
to
:
Average rate of change in area is

(b)
Instantaneous rate of change when
:
Instantaneous rate of change of
with respect to
when
is defined as
.
Area of the circle is
.
Differentiate on each side with respect to
.


.
Instantaneous rate of change of area at
is
.

.
(c) The circumference of the circle is
, where
is the radius of the circle.
consider
.
Therefore rate of change of area with respect to its radius is equal to the its circumference.
If the radius is increased by an amount of
.
Geometrical view of circle with an increased radius
.
Therefore area of circle with increased radius of
is
.
If
is small, then we can approximate
.
Rate of change of area of new circle is with respect to radius can be considered as
(a)
(i) Average rate of change if radius
changes from
to
is
.
(ii) Average rate of change if radius
changes from
to
is
.
(iii) Average rate of change if radius
changes from
to
is
.
(b) Instantaneous rate of change when
is
.
(c) The rate of change of area with respect to its radius is equal to the its circumference.

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