# Thread: Difficulty with Optimisation (Maxima and Minima) and Integration Problems

1. ## Difficulty with Optimisation (Maxima and Minima) and Integration Problems

Hi there,

There are a few optimisation and integration problems I can't seem to solve. Please help me, exam is on Tuesday... (Don't have to solve it, just tell me how to solve the questions )

1. P is a moveable point on the line y=7-x. Find the coordinates of P when it is closest to the point (2,4).
2. A small boat moving at x km/h uses fuel at a rate that is approximated by the function q=8+x^2/50, where q is measured in litres/hour. Determine the speech of the boat for which the amount of fuel used for any given journey is least.
3. An 8m high fence stands 3m from a large vertical wall. Find the length of the shortest ladder that will reach over the fence to the wall behind, as shown in the diagram.
4. A motorbike is 30km directly west of a car and begins moving east at 90km/h. At the same instant, the car begins moving north at 60km/h. What will be the minimum distance between the vehicles.

This is the integration question:
5. The cross-section of a channel is parabolic. It is 3 metres wide at the top and 2 metres deep. Find the depth of water, to the nearest cm, when the channel is half full. (For this one, I got that when the channel is half-full, 2 squared metres of the cross-section need to be covered, but I can seem to solve the question. Do I have to integrate with respect to y?)

Thank you so much for your help

2. ## Re: Difficulty with Optimisation (Maxima and Minima) and Integration Problems

I will help with first two problems, but before this here is the link to get a better touch of optimization problems.
1.P is a moveable point on the line y=7-x. Find the coordinates of P when it is closest to the point (2,4).
Since P is a point on line y=7-x, then coordinates of point P are (x,7-x).
Now, calculate distance between point P and point (2,4):
$d=\sqrt{(x-2)^2+(7-x-4)^2}=\sqrt{x^2-4x+4+9-6x+x^2}=\sqrt{2x^2-10x+13}$
Clearly if we find minimum of d then it will also be minimum for $d^2$. Therefore, for simplicity we will find minimum of $f(x)=d^2=2x^2-10x+13$
Find derivative: $f'(x)=4x-10$
Set f'(x)=0: 4x-10=0 or x=2.5.
Then y=7-x=7-2.5=4.5

Therefore, the closest point is P=(2.5,4.5). $d=\sqrt{(2.5-2)^2+(4.5-4)^2}=0.5\sqrt{2}$

2.A small boat moving at x km/h uses fuel at a rate that is approximated by the function q=8+x^2/50, where q is measured in litres/hour. Determine the speech of the boat for which the amount of fuel used for any given journey is least.

This means that if boat is moving at x km/h then it uses $f(x)= \frac{q}{x}=\frac8x+\frac{x}{50}$ litres per kilometer.
We must minimize this quantity: $f'(x)=-\frac{8}{x^2}+\frac{1}{50}$
Set f'(x)=0: $-\frac{8}{x^2}+\frac{1}{50}=0$ or $x^2=400$ or x=20 and x=-20. x=-20 is impossible because speed cannot be negative.
Therefore, only x=20 is applicable. Now we need to check that x=20 is indeed minimum.
For this we will use second derivative test: since $f''(x)=\frac{16}{x^3}$ then $f''(20)=\frac{16}{8000}>0$ therefore x=20 is indeed minimum.

3. ## Re: Difficulty with Optimisation (Maxima and Minima) and Integration Problems

Originally Posted by chessweicong
Hi there,

There are a few optimisation and integration problems I can't seem to solve. Please help me, exam is on Tuesday... (Don't have to solve it, just tell me how to solve the questions )

1. P is a moveable point on the line y=7-x. Find the coordinates of P when it is closest to the point (2,4).
2. A small boat moving at x km/h uses fuel at a rate that is approximated by the function q=8+x^2/50, where q is measured in litres/hour. Determine the speech of the boat for which the amount of fuel used for any given journey is least.
3. An 8m high fence stands 3m from a large vertical wall. Find the length of the shortest ladder that will reach over the fence to the wall behind, as shown in the diagram.
4. A motorbike is 30km directly west of a car and begins moving east at 90km/h. At the same instant, the car begins moving north at 60km/h. What will be the minimum distance between the vehicles.

This is the integration question:
5. The cross-section of a channel is parabolic. It is 3 metres wide at the top and 2 metres deep. Find the depth of water, to the nearest cm, when the channel is half full. (For this one, I got that when the channel is half-full, 2 squared metres of the cross-section need to be covered, but I can seem to solve the question. Do I have to integrate with respect to y?)

Thank you so much for your help
For question 1 let the point be (a,b) It will be closest to (2,4) when the line joining (a,b) to (2,4) is perpendicular to y=7-x
which has gradient -1 So we require a gradient of +1 So (b-4)/(a-2)=1 Also (a,b) is on y=7-x So b=7-a Solve these two equations

4. ## Re: Difficulty with Optimisation (Maxima and Minima) and Integration Problems

Thank you guys so much Is there anyone who knows how to do the other ones? I tried but they are all too difficult for me

5. ## Re: Difficulty with Optimisation (Maxima and Minima) and Integration Problems

3. Let x be distance between bottom of ladder and fence. Let y be the difference between height of fence and height of top of ladder. Let L be the length of ladder.
Then by Pythagorean theorem: $L^2=(x+3)^2+(y+8)^2$
From similar triangles $\frac{x}{8}=\frac{x+3}{y+8}$
From this $y+8=\frac{8(x+3)}{x}$
So, $L^2=(x+3)^2+(\frac{8(x+3)}{x})^2$
Or $L^2=(x+3)^2+(8+\frac{24}{x})^2$
Note, that L and $L^2$ share same minimum.
So, we will minimize $f(x)=L^2$:
$f'(x)=2(x+3)+2(8+\frac{24}{x})(\frac{-24}{x^2})$
I think next steps are clear.
4. Let's take origin to lie where car stands and let t be the time.
Then position of car at any time t is 60t and position of motorbike is 90t-30.
Distance between the by pythagorean theorem is $d^2=(60t)^2+(90t-30)^2$
Again we minimize $f(t)=d^2$

$f'(t)=2*60t*60+2(90t-30)*90$
Set f'(t)=0: $2*60t*60+2(90t-30)*90=0$ or $t=\frac{3}{13}$
Minimum distance is $d=\sqrt{(60t)^2+(90t-30)^2}=\sqrt{(60*\frac{3}{13})^2+(90*\frac{3}{13}-30)^2}=\frac{60}{\sqrt{13}}$ kilometers

6. ## Re: Difficulty with Optimisation (Maxima and Minima) and Integration Problems

Thank you so much!