2u maths - need help with question (1 Viewer)

[clry]

http://i52.tinypic.com/2zp4jer.jpg

I get part one, obviously. But the second part of the question, can someone help please?

Thanks!

 

[zoepi]

if it's a max/min problem don't you just find second derivative, if it's >0 it's a min etc?
just a thought, don't take my word for it haha, good luck.

 

[funnytomato]

first derivative f'(x)=0 at MIN/MAX
so just differentiate(chain rule) f(x) to get f'(x), and solve f'(x) = 0 for x value(s), say x=n if one of the solution(s) i.e. f'(n)=0

then differentiate f'(x) to get the second derivative f''(x)
evaluate f''(n), if it's positive, the point (n,f(n))a MIN
if -ve, it's a MAX

 

[SpiralFlex]

Let's see.

We know that the river is 500 metres wide.

Hence by pythagoras,





Hence the total time of AB is

Now, BC is

The time is




Let us differentiate.







For maximum or minimum,












Editing. Someone check my algebra. I get an negative squared answer. Find the solution and then substitute into the original equation to find the time taken.

After that you would take the second derivative.

If the second derivative is greater than zero it is a minimum turning point. If it is less than zero, it is a maximum turning point. Is there something wrong with this question? Or am I on too much Maccas. Okay I don't think I am crazy.

 

[SpiralFlex]

Okay, I am 99.99% sure there's something wrong with this question.

 

[Alkanes]

Check in maths in focus book.

 

[SpiralFlex]

It certainly has a glossy surface. But I cannot compute what textbook it is. The number 10 looks to be coloured. Could it be the second edition of MIF? If you look carefully there is an in print of a cyclic quadrilateral with a line coming out of it, presumably the cross section of some pole. Rules out Cambridge; the shape of the 10 is too round. The Coroneos book is also ruled out; the font is different. So the only book that I know off with a circle imprinted at the back of a maximum/minimum question is -

It's the MIF 2U book.

 

[Alkanes]

Its the HSC 2U MIF one

 

[clry]

It certainly has a glossy surface. But I cannot compute what textbook it is. The number 10 looks to be coloured. Could it be the second edition of MIF? If you look carefully there is an in print of a cyclic quadrilateral with a line coming out of it, presumably the cross section of some pole. Rules out Cambridge; the shape of the 10 is too round. The Coroneos book is also ruled out; the font is different. So the only book that I know off with a circle imprinted at the back of a maximum/minimum question is -

It's the MIF 2U book.

Correcto, and being the classy text book that it is, it only has the answer for part 1, and not part 2, which was where I was stumped :<

 

[SpiralFlex]

Correcto, and being the classy text book that it is, it only has the answer for part 1, and not part 2, which was where I was stumped :<

The question is not possible. I have checked my algebra.

 

[clry]

Cheers for the process, I followed your work which made sense, so at least I got the process out of it. Thanks!

 

[SpiralFlex]

Cheers for the process, I followed your work which made sense, so at least I got the process out of it. Thanks!

These are types of questions you will see in the 2U paper. You must be able to know how to do these.

 

[clry]

These are types of questions you will see in the 2U paper. You must be able to know how to do these.

Yeah, I understand what to do now.. I could do the previous ones, it's just this one, I got some ridiculous answer.

 

[Drongoski]

The question is not possible. I have checked my algebra.

I think I worked on this question a few years ago. I recall some difficulties with the question. I think I broke it down into a few cases and treated each separately. Cannot remember where I kept my write-up.

 

[SpiralFlex]

I think I worked on this question a few years ago. I recall some difficulties with the question. I think I broke it down into a few cases and treated each separately. Cannot remember where I kept my write-up.

Post solutions when you find it.

 

[ohexploitable]

ummm lol you guys, especially spiralflex,

the derivative obviously can't be zero, so 't' will be proportional to 'x', you can even think of it as a linear relationship, in this case the max/min of 't' lies in the extremes of the domain of 'x'

i.e. max at x=0, min at x=7

 

[ohexploitable]

for example, if:



then obviously there are no stationary pts but there still is a max/min for the given question, min at x=0, max at x=7 in my example, extremes of the domain

 

[ohexploitable]

if u still don't understand:

http://www.wolframalpha.com/input/?i=t=+sqrt(x^2+0.25)/5+++(7-x)/4+from+0+to+7

vertical axis is 't', horizontal is 'x'