The Student Room Group

Differentiatio help please.....

Find dy/dx of y=(2x+3)(x+5)^2
Okay, so I'm using product rule and this is what i've done so far:

(2x+3)(x+5)^2
2 [x+5]^2 + 2x+3 (2) (x+5) (1)
=2 [x+5]^2 + 2*2x+3 (x+5)

What do I do next, show the steps please?
Reply 1
Original post by Nkhan
Find dy/dx of y=(2x+3)(x+5)^2
Okay, so I'm using product rule and this is what i've done so far:

(2x+3)(x+5)^2
2 [x+5]^2 + 2x+3 (2) (x+5) (1)
=2 [x+5]^2 + 2*2x+3 (x+5)

What do I do next, show the steps please?


There are a number of ways you could do this. You could expand the 3 brackets and then it'd just be straight differentiation of a polynomial. You could use the product rule

ddxuv=dudxv+udvdx\frac{d}{dx}uv = \frac{du}{dx}v + u\frac{dv}{dx}

Where

u=2x+3u = 2x+3

and

v=(x+5)2v = (x+5)^2

Or you could do the product rule of 3 functions.

ddxuvw=dudxvw+udvdxw+uvdwdx\frac{d}{dx}uvw = \frac{du}{dx}vw + u\frac{dv}{dx}w + uv\frac{dw}{dx}

Where

u=2x+3u = 2x+3

and

v=w=x+5v = w = x+5


A lot of people seem to forget to apply the chain rule, so the best method to ensure you do it correctly would be either to multiply it all out, or the last method, product rule of 3 functions.

Could you rewrite what you've got up to? What you have done seems to be correct, although don't forget you're multiplying the 2 from the (x+5)2(x+5)^2 with the entire function, not just the 2x ( you didn't put brackets around it).
(edited 13 years ago)
Reply 2
2(x+5)^2 + 2(2x+3)(x+5)

2(x+5)[x+5+2x+3]

2(x+5)(3x+8)
Reply 3
Original post by Noble.
There are a number of ways you could do this. You could expand the 3 brackets and then it'd just be straight differentiation of a polynomial. You could use the product rule

ddxuv=dudxv+udvdx\frac{d}{dx}uv = \frac{du}{dx}v + u\frac{dv}{dx}

Where

u=2x+3u = 2x+3

and

v=(x+5)2v = (x+5)^2

Or you could do the product rule of 3 functions.

ddxuvw=dudxvw+udvdxw+uvdwdx\frac{d}{dx}uvw = \frac{du}{dx}vw + u\frac{dv}{dx}w + uv\frac{dw}{dx}

Where

u=2x+3u = 2x+3

and

v=w=x+5v = w = x+5


A lot of people seem to forget to apply the chain rule, so the best method to ensure you do it correctly would be either to multiply it all out, or the last method, product rule of 3 functions.

Could you rewrite what you've got up to? What you have done seems to be correct, although don't forget you're multiplying the 2 from the (x+5)2(x+5)^2 with the entire function, not just the 2x ( you didn't put brackets around it).

Okay thanks :smile: And yeah the simplifying but is the difficult bit for me, I just dont no when and what to cancel out :|
But thanks for ur help
Reply 4
Original post by Pheylan
2(x+5)^2 + 2(2x+3)(x+5)

2(x+5)[x+5+2x+3]

2(x+5)(3x+8)

How did you cancel out? what bits cancelled out each other? Thats the hardest part for me :|
Reply 5
Original post by Nkhan
How did you cancel out? what bits cancelled out each other? Thats the hardest part for me :|


factorise 2(x+5) from both terms, this leaves you with x+5 in the first term and 2x+3 in the second. x+5+2x+3 = 3x+8
Reply 6
Original post by Nkhan
Okay thanks :smile: And yeah the simplifying but is the difficult bit for me, I just dont no when and what to cancel out :|
But thanks for ur help


dydx=2(x+5)2+2(2x+3)(x+5)\frac{dy}{dx} = 2(x+5)^2 + 2(2x+3)(x+5)

You can take out a factor of 2 and (x+5)(x+5) from both.

dydx=2(x+5)[(x+5)+(2x+3)]\frac{dy}{dx} = 2(x+5)[(x+5) + (2x+3)]
Reply 7
Original post by Pheylan
factorise 2(x+5) from both terms, this leaves you with x+5 in the first term and 2x+3 in the second. x+5+2x+3 = 3x+8

Ahh right, thanks so much :smile:

Quick Reply

Latest