maximum and minimum problems
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Maximum and minimum problems
Calculus AS 3.6
A rectangular block is constructed so that its length is twice its breadth. Find the least possible surface area of the block if its volume is 72
Step 1: Draw a diagram and label it
“its length is twice its breadth”
x2x
h
Step 2: Write the equation for the relationship given
its volume is 72
x2x
h
Step 3: Write the equation for what you are trying to maximise or minimise
“Find the least possible surface area”
x2x
h
Step 4: Use the information to write this equation in one variable only
“Find the least possible surface area”
Step 5: Differentiate = 0
“Find the least possible surface area”
Step 6: Answer the question
“Find the least possible surface area”
IF the size of a parcel sent through the post is limited by the fact that the sum of its length and girth must not exceed 2 m, find the volume of the largest rectangular parcel with a square base which may be posted.
1. Draw a diagram
xx
y
2. Write the equation for the relationship given
xx
y
3. Write the equation for what you are trying to maximise or minimise
xx
y
find the volume of the largest rectangular parcel
4: Use the information to write this equation in one variable only
xx
y
find the volume of the largest rectangular parcel
5: Differentiate = 0
find the volume of the largest rectangular parcel
6: Answer the question
find the volume of the largest rectangular parcel
Don’t forget units
A feeding trough is made from three pieces of metal welded
together, one rectangle of size 3k cm by 5k cm and two
trapeziums. The length of the trough is 5k cm and its cross-
section is shown in the diagram. (k is constant)
k k
k
Find the maximum volume of the trough.
(5k long)
k k
k
Diagram is given and we only need to use 1 variable, θ
(5k long)
k k
k
(5k long)
k k
k
(5k long)
k
k k
k
(5k long)
k
k k
k
(5k long)
k
k k
k
(5k long)
k
k k
k
(5k long)
k
k k
k
k
k k
k
Find the maximum volume of the trough.
k
A circle of radius l is used to form a cone. Find the angle of the sector that forms a cone of the maximum volume.
Think about how the cone is created
θ
l
l
l
The arc length is the same as the circumference of the rim of the cone.
θ
l
l
l
r
Using Pythagoras’ theorem
θ
l
l
l
r
h
Volume of the cone =
θ
l
l
l
r
h
Volume of the cone =
Substituting
Clean it up
Differentiate using product rule
Differentiate using product rule
Would give a minimum
Differentiate using product rule
If total revenue for a firm is given by
where x is the number of units sold, find the number of units that must be sold to maximize revenue.
The number of units that must be sold to maximize revenue is 40.
A travel agency will plan a group tour for groups of 25 or larger. If the group contains exactly 25 people, the charge is $300 per person. However, each person’s cost is reduced by $10 for each additional person above the 25. What size group will produce the largest revenue for the agency?
A travel agency will plan a group tour for groups of 25 or larger. If the group contains exactly 25 people, the charge is $300 per person. However, each person’s cost is reduced by $10 for each additional person above the 25. What size group will produce the largest revenue for the agency?
R = number of people x cost
A travel agency will plan a group tour for groups of 25 or larger. If the group contains exactly 25 people, the charge is $300 per person. However, each person’s cost is reduced by $10 for each additional person above the 25. What size group will produce the largest revenue for the agency?
R = number of people x cost
A travel agency will plan a group tour for groups of 25 or larger. If the group contains exactly 25 people, the charge is $300 per person. However, each person’s cost is reduced by $10 for each additional person above the 25. What size group will produce the largest revenue for the agency?
R = number of people x cost
Group size 22 or 23 for revenue $7560
Suppose the production capacity for a certain commodity cannot exceed 30. If the total profit function for this commodity is
where x is the number of units sold, find the number of items that will maximize profit.
Suppose the production capacity for a certain commodity cannot exceed 30. If the total profit function for this commodity is
where x is the number of units sold, find the number of items that will maximize profit.
Suppose the production capacity for a certain commodity cannot exceed 30. If the total profit function for this commodity is
where x is the number of units sold, find the number of items that will maximize profit.Check end point x = 30, P = $26,800
End point x = 30, P = $26,800 gives the maximum
A man at a point A on the shore of a circular lake with radius 2 km wants to be at point C diametrically opposite A on the other side of the lake in the shortest possible time. The ratio of his walking speed to his rowing speed is 2:1.At what angle to the diameter should he row?
Check end points
Time to row across
A C
4 km
Time to walk around
A C
4 km
Time to row and then walk:
A C
4 km
DR
θ
Dw
Angle in a semi-circle means we have a rt angled triangle
A C
4 km
DR
θ
Length of the arc
A C2 km
Dw
θ 2θ
Differentiate
Differentiate and solve
Differentiate and solve
Differentiate and solve
Walking around the lake takes the least time
My boat travels at 30 km/hr and, because I never speed, I travel at 50 km/hr by car along the coast. At what point should I keep the car to minimize the time travelling to town.
8 km
town48 km
Evaluate the end points: My boat travels at 30 km/hr
8 km
town48 km
Evaluate the end points: Boat then car
8 km
town48 km
Evaluate: Boat to x, then car
8 km
town(48 – x) kmx
Evaluate: Boat to x, then car
8 km
town(48 – x) kmx
Evaluate: Boat to x, then car
Evaluate: Boat to x, then car
8 km
town(48 – x) kmx
The cost of fuel per hour of running a jet aircraft, $C, is a function of its cruising velocity v km/hr.
Find the most economical cruising velocity for a journey of 6000 km.
The cost of fuel per hour of running a jet aircraft, $C, is a function of its cruising velocity v km/hr.
Find the most economical cruising velocity for a journey of 6000 km.
The cost of fuel per hour of running a jet aircraft, $C, is a function of its cruising velocity v km/hr.
Find the most economical cruising velocity for a journey of 6000 km.
The cost of fuel per hour of running a jet aircraft, $C, is a function of its cruising velocity v km/hr.
Find the most economical cruising velocity for a journey of 6000 km.
The cost of fuel per hour of running a jet aircraft, $C, is a function of its cruising velocity v km/hr.
Find the most economical cruising velocity for a journey of 6000 km.
The total cost per hour of running a ship while on a voyage is
Where v is the constant speed for the voyage in km per hour. If the ship makes a voyage of 2000 km, find the speed which gives the most economical cost for the voyage.
A trapezium is inscribed in a semicircle of radius r so that one side of the trapezium is on the diameter of the semicircle. Find the maximum area of the trapezium, in terms of r.
Find the maximum area of the trapezium, in terms of r.
Find the maximum area of the trapezium, in terms of r.
Find the maximum area of the trapezium, in terms of r.
Find the maximum area of the trapezium, in terms of r.
A rectangle is drawn as shown. The equation of the parabola is
Find the greatest area that this rectangle can have.
A rectangle is drawn as shown. The equation of the parabola is
Find the greatest area that this rectangle can have.
A rectangle is drawn as shown. The equation of the parabola is
Find the greatest area that this rectangle can have.
A cone has a height of 15 cm and radius of base of 9 cm.
Another smaller cone is inscribed in the cone. Its vertex is at the centre of the base of the larger cone. The base of the smaller cone is parallel to the base of the larger cone.
Find h and r, the dimensions of the smaller cone, so that it has maximum volume.
State the maximum volume of the smaller cone.
Find h and r, the dimensions of the smaller cone, so that it has maximum volume.State the maximum volume of the smaller cone.
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