Asked by Tom

find the first and second derivative of:

f(x) = (x+2)/e^x


I get to:

f'(x) = e^x - (x+2)(e^x) / (e^x)^2

I'm stuck at that

Answers

Answered by Steve
I think the quotient rule gets bogged down here. Use the product rule with a negative exponent:

y = (x+2)e<sup>-x</sup>

y' = e<sup>-x</sup> - (x+2)e<sup>-x</sup>
= -(x+1)e<sup>-x</sup>

y'' = -e<sup>-x</sup> + (x+1)e<sup>-x</sup>
= xe<sup>-x</sup>

However, using your quotient rule, factor out the ex top and bottom:

e<sup>x</sup>(1 - (x+2))/e<sup>2x</sup>
= -(x+1)/e<sup>x</sup>

Do it again, and you wind up where I did.
Answered by Tom
Tnx for the quick answer, but I still have a question:

why is it - in y' = e^-x (-) (x+2)e^-x
instead of a + ?
Answered by Tom
and how do you get (x+1)?
Answered by Tom
oh I get it now, tnx!
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