long question , confusing , please check them for me,urgent!!!

Feb 2016
11
0
uk
Same as the title, please figure out what is wrong for my working ,,
questions:
1)A helicopter takes off from a point 120m away from an abserver located on the ground, and rises vertically at a constant rate of 2.5m/s.At what rate is the elvation angle of the observer's ine of sight to te helipoter changeing ehrn the helicopter is 90m above the ground (Assume the height from the ground, of the observe's sight level is neglible)?
my working::::::The tangent of the angle at any time is given by:

tan(θ) = y / 120 differentiating this with respect to time, t, we have

sec^2(θ) dθ/dt = (1/120) dy/dt (1)

When the helicopter is 90m above the ground, θ is given by

arctan(90/120) = about .6435 rads ....so....filling in the missing pieces in (1), we have

sec^2(.6435) dθ/dt = (1/120m) (2.5m/s)

dθ/dt = (1/120m) (2.5m/s)/ sec^2(.6435) = about .01334 rads/s



2.)A piece of wire of length 100cm is cut into two parts.One parts is bent into a square, while the other part is bent into a circle.The length of a side of the sqaure is x cm.Find the value of x which gives the least total area.What is the least total area?

my working:::::
2) The area of the square = x^2


The perimeter of the circle = 100 - 4x = 2*pi*r

Therefore.....the radius of the circle is [50 -2x] /pi

So....the total area is given by :

A = x^2 + pi ( [50 -2x] /pi )^2 = x^2 + (1/pi) *(50-2x)^2

Take the derivative of this and set it to 0

A' = 2x - 2* (2/pi) [ 50 - 2x] =

2x + 8[x -25] /pi = 0

x + 4[x - 25]/ pi = 0

pi*x + 4x - 100 =0

x (pi + 4) = 100

x = 100 / (pi + 4) = about 14.002 cm

The second derivative =

1 + 4/pi .....since this is always positive, the curve is concave up at all points....so x = 14.002cm is a minimum


The minimum total area is about 350.062 cm^2
 
Jun 2008
1,389
513
Illinois
Your work look correct to me - why do you think something is wrong?
 

skeeter

MHF Helper
Jun 2008
16,217
6,765
North Texas
what makes you think anything is wrong?
 
Feb 2016
11
0
uk
As I really bad in maths,so I want to confirm that I am at the right approach.thanks