The Student Room Group
Reply 1
x=0, y=0 (so it goes through the origin)

x -> infinity, y -> 0 (since e^-x will decrease at a faster rate than x increases)
x -> -(infinity), y -> -(infinity), since e^-x is now +ve (and very large), and x is negative.

dy/dx = e^(-x) - x.e^(-x) = 0 (for turning point)

e^(-x) [1-x] = 0

x=1 for a turning point.
x=1, y = 1/e

d^2y/dx^2 = -e^(-x)[1-x] - e^(-x)

Substituting x=0, you get d^2y/dx^2 = -e^(-1) < 0 => Maximum at (1, 1/e)

I think this is what it looks like. Should be a smooth curve going through the origin, peaking at (1,1/e) and levelling off to y=0.
Reply 2
mockel


I think this is what it looks like. Should be a smooth curve going through the origin, peaking at (1,1/e) and levelling off to y=0.


Your sketch looks at least as good as the display on my TI-83, and just as accurate!

Aitch
Reply 3
Hehehe. Nice! :smile:
Reply 4
isn't there also a turning point at x=-1?
Reply 5
nas7232
isn't there also a turning point at x=-1?

I can't see why :confused:

Unless I've missed something. Have I?
No there isn't.
Reply 7
ah, i did a stupid mistake for the values of x<0

can anyone show me how

(x^2).(e^-x)

looks like, i got 1 stationary point (havent got my work on me so i can tell you peeps, i think i got it to be 2 or soemthing, either 2,1/2 or -2). Thanks :smile:
Reply 8
x=0, y=0
x -> infinity , y -> 0
x -> -(infinity) , y -> infinity

dy/dx = 2x.e^(-x) - (x^2).e^(-x) = 0

xe^(-x)[(2-x)] = 0

x=0, or x=2 for stationary points.
x=2, y = 4e^(-2)


d^2/dx^2 = -e^(-x)[2x - x^2] + e^(-x)[2-2x]

x=0, d^2y/dx^2 = 2 >0 => Minimum at (0,0)

x=2, d^2y/dx^2 = -2e^(-2) <0 => Maximum at (2, 4e^-2)

The graph should look like this:
Reply 9
nice1 :cool: