C3 Product Rule Help

Hey, i was looking for an insight into where ive gone wrong on the folllowing question:

Differentiate y=(x+2)e^-x

i did the product rule and got
Y=uv

u= x+2
du/dx= 1

v= e^-x
dv/dx= -e^-x

so dy/dx= e^-x + -e^-x(x+2)

but the mark scheme says its
-e^-x(x+1)

thanks for the read, any help is appreciated

Reply 1

Blazy

8

Original post by TheGreat1

Hey, i was looking for an insight into where ive gone wrong on the folllowing question:

Differentiate y=(x+2)e^-x

i did the product rule and got
Y=uv

u= x+2
du/dx= 1

v= e^-x
dv/dx= -e^-x

so dy/dx= e^-x + -e^-x(x+2)

but the mark scheme says its
-e^-x(x+1)

thanks for the read, any help is appreciated

Your answer is correct - you should simplify it down. Can you see a factor which you can take out?

EDIT: Maybe not. -e^-x(x+2) - this bit isn't right.
EDIT2: Gah shutup Blazy.

(edited 11 years ago)

Reply 2

TenOfThem

18

Original post by TheGreat1

Hey, i was looking for an insight into where ive gone wrong on the folllowing question:

Differentiate y=(x+2)e^-x

i did the product rule and got
Y=uv

u= x+2
du/dx= 1

v= e^-x
dv/dx= -e^-x

so dy/dx= e^-x + -e^-x(x+2)

but the mark scheme says its
-e^-x(x+1)

thanks for the read, any help is appreciated

What happens when you take out the common factor of

-e^{-x}

Reply 3

steve2005

17

Original post by TheGreat1

Hey, i was looking for an insight into where ive gone wrong on the folllowing question:

You can check that your answer is correct or otherwise by using your calculator. Set x = 1.5 or something and see what happens.

There is usually no need for YOUR answer to look exactly like the answer in the mark scheme.