# Higher Maths: OptimisationWatch

#1
Hey,
I've been trying to get this optimisation question.
I've got the answer to (b), but cannot for the love of me get part (a)

Could someone give me some hints as to how you get it?

Thanks!
0
9 years ago
#2
try to get the height in terms of x.

if you're still struggling:
Spoiler:
Show
inner surface area = 2 sides + 2 sides + bottom
12=2(2xh)+2(xh) + 2x^2

then V=lbh
0
9 years ago
#3
For part b, do you just differentiate?
0
#4
try to get the height in terms of x.

if you're still struggling:
Spoiler:
Show
inner surface area = 2 sides + 2 sides + bottom
12=2(2xh)+2(xh) + 2x^2

then V=lbh
Hey,
I managed to get the surface area earlier... but just can't work out the next bit
Still can't get it How I hate the first bit of these questions!
0
#5
(Original post by CT_Scan)
For part b, do you just differentiate?
Yeah, then nature table.
0
9 years ago
#6
(Original post by FuturePilot)
Yeah, then nature table.

Yeah, thanks for reassuring me. I am sorry but I cannot help you with the first part, have you had your prelim yet? Mines is tomorrow.
0
9 years ago
#7
Part b:

Differentiate

Nature Table

Equate to 0

Then Solve

Methinks
0
9 years ago
#8
(Original post by FuturePilot)
Hey,
I managed to get the surface area earlier... but just can't work out the next bit
Still can't get it How I hate the first bit of these questions!
rearrange the surface area equation to get (6-x^2)/3x=h

v=lbh
v=2x^2 x (6-x^2/3x)

v=2x^2/3x x (6-x^2)

v=2x/3 (6-x^2) as required.
0
#9
(Original post by CT_Scan)
Yeah, thanks for reassuring me. I am sorry but I cannot help you with the first part, have you had your prelim yet? Mines is tomorrow.
No bother. Oh, good luck! Mine's on Friday
0
9 years ago
#10
Could someone help me with this one?

f(x) = 4x^2 - 14x +24

It was in our prelim and we had to find the answer which minimizes the function.

I think I did it the right way but I don't know if I got the right answer?
0
9 years ago
#11
(Original post by Lulope)
Could someone help me with this one?

f(x) = 4x^2 - 14x +24

It was in our prelim and we had to find the answer which minimizes the function.

I think I did it the right way but I don't know if I got the right answer?
f'(x) = 8x - 14

f'(x) = 0 at sps,
8x = 14
x = 7 / 4

(this is clearly going to be the minimum, since there aren't any other sps!)

f(7 / 4) = 4(49/16) - 14(7/4) + 24 = 49 / 4 - 49 / 2 + 24 = 0.25(49 - 98 + 96) = 47 / 4

is the minimum tp of f.

(The way you've worded it sounds like you only needed x = 7 / 4 as the answer, though)
0
9 years ago
#12
(Original post by TheUnbeliever)
f'(x) = 8x - 14

f'(x) = 0 at sps,
8x = 14
x = 7 / 4

(this is clearly going to be the minimum, since there aren't any other sps!)

f(7 / 4) = 4(49/16) - 14(7/4) + 24 = 49 / 4 - 49 / 2 + 24 = 0.25(49 - 98 + 96) = 47 / 4

is the minimum tp of f.

(The way you've worded it sounds like you only needed x = 7 / 4 as the answer, though)
Thank you!

I have a horrible feeling I integrated instead of differentiating
0
2 years ago
#13
-differentiate and make equal to zero for x.
-find the second derivative (so differentiate again)
-substitute in x to the second derivative and if your answer is negative, it gives a Maximum
.
0
2 years ago
#14
7 years later ... they have probs got their degree now hahah
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