Transcript Limits and Derivatives

Chapter 3 – Differentiation Rules
3.9 Related Rates
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Section 3.9 Related Rates
Related Rates

If we are pumping air into a balloon, both the
volume and the radius of the balloon are
increasing and their rates of increase are
related to each other.

But it is much easier to measure directly the
rate of increase of the volume than the rate of
increase of the radius.
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Section 3.9 Related Rates
Related Rates
In related rate problems, the idea is to compute
the rate of change of one quantity in terms of
the rate of change of another quantity (which
may be more easily measured).
The procedure is to find an equation that relates
the two quantities and then use the Chain Rule
to differentiate both sides with respect to time.
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Section 3.9 Related Rates
Book Example 1 – pg. 248 # 1
If V is the volume of a cube with edge length x
and the cube expands as time passes, find
dV/dt in terms of dx/dt.
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Section 3.9 Related Rates
Book Example 2 – pg. 248 # 2
a)
If A is the area of a circle with radius r and
the circle expands as time passes, find dA/dt
in terms of dr/dt.
b)
Suppose oil spills from a ruptured tanker and
spreads in a circular pattern. If the radius of
the oil spill increases at a constant rate of
1 m/s, how fast is the area of the spill
increasing when the radius is 30 m?
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Section 3.9 Related Rates
Book Example 3 – pg. 248 # 5
A cylindrical tank with radius 5 m is being filled
with water at a rate of 3 m3/min. How fast is
the height of the water increasing?
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Section 3.9 Related Rates
Example 4
If
z 2  x2  y 2
dx
2
dt
dy
3
dt
then find
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dz
dt
when x=5 and y=12.
Section 3.9 Related Rates
Book Example 5 – Pg. 248 #11
A plane flying horizontally at an altitude of 1 mile
and a speed of 500 mph passes directly over a
radar station. Find the rate at which the distance
from the plane to the station is increasing when
it is 2 mi away from the station.
a)
b)
c)
d)
e)
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What quantities are given in the problem?
What is the unknown?
Draw a picture of the situation for any time t.
Write an equation that relates the quantities.
Finish solving the problem.
Section 3.9 Related Rates
Book Example 6 – pg. 248 #14
At noon, ship A is 150 km west of ship B. Ship A is
sailing east at 35 km/h and ship B is sailing north
at 25 km/h. How fast is the distance between the
ships changing at 4:00 pm?
a)
b)
c)
d)
e)
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What quantities are given in the problem?
What is the unknown?
Draw a picture of the situation for any time t.
Write an equation that relates the quantities.
Finish solving the problem.
Section 3.9 Related Rates
Book Example 7 – pg. 249 # 23
Water is leaking out of an inverted conical tank
at a rate of 10,000 cm3/min at the same time
that water is being pumped into the tank at a
constant rate. The tank has height 6 m and the
diameter of the top is 4 m. If the water level is
rising at a rate of 20 cm/min when the height
of the water is 2 m, find the rate at which the
water is being pumped into the tank.
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Section 3.9 Related Rates
Book Example 8 – pg. 249 # 26
A swimming pool is 20 ft wide, 40 ft long, 3 ft deep at
the shallow end, and 9 ft deep at its deepest point. A
cross-section is shown below. If the pool is being
filled at a rate of 0.8 ft3/min, how fast is the water
level rising when the depth at the deepest point is 5
ft?
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Section 3.9 Related Rates
Book Example 9 – pg. 249 # 27
Gravel is being dumped from a conveyor belt at a rate
of 30 ft3/min, and its coarseness is such that it forms
a pile in the shape of a cone whose base diameter
and height are always equal. How fast is the height
of the pile increasing when the pile is 10 ft high?
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Section 3.9 Related Rates
Book Example 10 – pg. 249 # 35
If two resistors with resistance R1 and R2 are connected in
parallel (see figure), then the total resistance R,
measured in ohms () is given by
1 1
1
 
R R1 R2
If R1 and R2 are increasing at rates of 0.3 /s and 0.2 /s
respectively, how fast is R changing when R1=80 and
R2=100 ?
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Section 3.9 Related Rates
Book Example 11 – pg. 250 # 45
A runner sprints around a circular track of radius
100 m at a constant speed of 7 m/s. The
runner’s friend is standing at a distance of 200
m from the center of the track. How fast is the
distance between the friends changing when
the distance between them is 200 m?
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Section 3.9 Related Rates
Try these problems
A spotlight on the ground shines on a wall 12 m
away. If a man 2 m tall walks from the
spotlight toward the building at a speed of
1.6m/s, how fast is the length of his shadow on
the building decreasing when he is 4 m from
the building?
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Section 3.9 Related Rates
Try these problems
The altitude of a triangle is increasing at a rate of
1 cm/min while the area of the triangle is
increasing at a rate of 2 cm2/min. At what rate
is the base of the triangle changing when the
altitude is 10 cm and the area is 100 cm2.
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Section 3.9 Related Rates
Try these problems
A kite 100 ft above the ground moves
horizontally at a speed of 8 ft/s. At what rate is
the angle between the string and the horizontal
decreasing when 200 ft of string has been let
out?
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Section 3.9 Related Rates
Try these problems
A television camera is positioned 4000 ft from the base of
a rocket launching pad. The angle of elevation of the
camera has to change at the correct rate in order to keep
the rocket in sight. Also, the mechanism for focusing
the camera has to take into account the increasing
distance from the camera to the rising rocket. Let’s
assume the rocket rises vertically and its speed is 600
ft/s when it has risen 3000 ft.
a)
b)
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How fast is the distance from the television camera to the
rocket changing at that moment?
If the television camera is always kept aimed at the
rocket, how fast is the camera’s angle of elevation
changing a that same moment?
Section 3.9 Related Rates
Try these problems
A Ferris wheel with a radius of 10 m is rotating
at a rate of one revolution every 2 minutes.
How fast is the rider rising when his seat is
16m above ground level?
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Section 3.9 Related Rates