[CIE] All Pure Mathematics (P1, P2 and P3) doubts here !

[astarmathsandphysics]

One mo.

[astarmathsandphysics]

here

[curiousguy]

9709/13/m/j/10 Q8 (iii) please someone explain me this question.

[Most UniQue™]

here

Thanks a lot sir!

[urni.nath]

I have a doubt:
At a speed of S km per hour a car will travel y kilometers on each litre of petrol,where
y= 5 + 1/5 S - 1/800 S^2
Calculate the speed at which the car should be driven for maximum economy.

Any help is appreciated!
Thanks

[astarmathsandphysics]

dy/ds=0
1/5-s/400=0 so s=80

[urni.nath]

Thanks a lot! 

[astarmathsandphysics]

here

[urni.nath]

Thank you..sorry for the inconvenience but i have another doubt!

1) a rectangular sheet of metal measures 50cm by 40cm. Equal sides of side x cm are cut from each corner and discarded. The sheet is then folded up to make a tray of depth x cm. What is the domain of possible values of x ? Find the value of x which maximizes the capacity of the tray.
2)An open rectangular box is to be made with a square base, and its capacity is to be 4000cm^3 . Find the length of the side of the base when the amount of material used to make the box is as small as possible. (ignore "flaps")

Thank you so much for your help!

[astarmathsandphysics]

When I get home

[astarmathsandphysics]

Both questions

[urni.nath]

Thank you again!

[astarmathsandphysics]

No probs

[Most UniQue™]

Why has this thread been combined with P2 and P3?

[Malak]

Why has this thread been combined with P2 and P3?

I have combined them because there were too many sticky threads and I thought it is confusing and besides these aren't threads which are always crowded so its better to have one thread for a subject rather then having 3 threads for each unit of a subject.