[gavinlee]

Good evening all,

I've come across a question which I simply don't understand! It's:

"For the curve with equation show that y is decreasing when


Help!!!

[Intriguing Alias]

(Original post by gavinlee)
Good evening all,

I've come across a question which I simply don't understand! It's:

"For the curve with equation show that y is decreasing when


Help!!!

Well, what does 'y is decreasing' mean in terms of calculus?

[raheem94]

(Original post by gavinlee)
Good evening all,

I've come across a question which I simply don't understand! It's:

"For the curve with equation show that y is decreasing when


Help!!!

Remember 'y' decreasing means negative gradient(i.e. dy/dx < 0).

[gavinlee]

yeah, y decreasing means a negative gradient, a decreasing function. But I don't know what else to say?

[TenOfThem]

(Original post by gavinlee)
yeah, y decreasing means a negative gradient, a decreasing function. But I don't know what else to say?

did you differentiate?

[gavinlee]

No, haven't differentiated.
differentiated isn't it's

[TenOfThem]

(Original post by gavinlee)
No, haven't differentiated.
differentiated isn't it's

You know you need gradient <0 so you need to differentiate

You now have

Can you not see what to do next?

[raheem94]

(Original post by gavinlee)
No, haven't differentiated.
differentiated isn't it's

When y is decreasing gradient is less than zero.

So

Now try to get the form given in the question.

[gavinlee]

The form from:

to:

?

[raheem94]

(Original post by gavinlee)
The form from:

to:

?

Remember, multiply or dividing by a negative number flips the inequality sign.

Multiply throughout by



Sorry for posting the answer, but as the exam is approaching, i didn't wanted to waste your too much time on a simple question.

[gavinlee]

Thanks for your help. I'll look at it again tomorrow, maybe I'll be able to understand it then.

Thanks

[raheem94]

(Original post by gavinlee)
Thanks for your help. I'll look at it again tomorrow, maybe I'll be able to understand it then.

Thanks

What is difficult to understand in it?

We know y decreases when its gradient is less than zero.

We just need to differentiate and then do some simple manipulation to get the answer.

[Maxima]

Have you tried drawing the actual curve on a graph? That way it is easier to visualise, you can then pinpoint the exact location where the gradient starts to decrease as chances are you've probably worked this out somewhere in the question before.

Sorry this answer is very vague, don't have a pen and paper near me and can't really work this out by looking at it, but was doing a past paper before and a question similar to this came up.

[gavinlee]

Thanks everyone. Looked at it this morning with a fresh pair of eyes and it totally made sense.