You are Here: Home

# maximum of this equation?

Announcements Posted on
Fancy a fiver? Fill in our quick survey and we’ll send you a £5 Amazon voucher 03-05-2016
Talking about ISA/EMPA specifics is against our guidelines - read more here 28-04-2016
1. need help finding the maximum value for y

y = (5.0) sin 120x + (7.0) sin 240x.
2. I'm a bit confused, is the equation ?
and do you mean you want the maximum x value or maximum y?
3. that's the equation, and got to find maximum y
4. Well you know that the gradient of the curve is . So where you have a stationary point. You can then substitute the x value of the stationary points into to determine the nature of the stationary points. If then the stationary point is a minimum. If the stationary point is a maximum.

Does that help?
5. I did that and got know where you still have the equation

600cos 120x + 1680 cos 240x = 0

which I have no idea how to solve
6. (Original post by stubear)
I did that and got know where you still have the equation

600cos 120x + 1680 cos 240x = 0

which I have no idea how to solve
Assuming up to that point is correct (sorry i havent done the workings myself yet) you could say let t=120x

then you have 600cos(t) + 1680cos(2t) = 0

could you then solve that by using the double angle formulae?

I'm stuck again

600cost + 1680cos^2(t)-1680sin^2(t) = 0

sorry if I'm being thick
8. hehe no it's not a problem just gotta keep practising and you'l get there

600cos(t) + 1680cos(2t) = 0

should use: cos(2t) = 2cos^2(t) - 1

so 600cos(t) + 3360cos^2(t) - 1680 = 0

then you have a quadratic, so you can substitute the values of a b and c (ax^2+bx+c) into the quadratic formula to find the value for cos(t). You should then be able to solve to find 't', remembering t=120x, so you can then find what x equals
9. (Original post by just george)
Well you know that the gradient of the curve is . So where you have a stationary point. You can then substitute the x value of the stationary points into to determine the nature of the stationary points. If then the stationary point is a minimum. If the stationary point is a maximum.

Does that help?
Just like to make a quick point re d2y/dx2.

What you say is correct for finding if it's a max or min but what if d2y/dx2 = 0? From memory, sometimes it's quicker to find points either side to determine max / min.

But do correct me if I've forgotten my calculus.
10. (Original post by just george)
hehe no it's not a problem just gotta keep practising and you'l get there

600cos(t) + 1680cos(2t) = 0

should use: cos(2t) = 2cos^2(t) - 1

so 600cos(t) + 3360cos^2(t) - 1680 = 0

then you have a quadratic, so you can substitute the values of a b and c (ax^2+bx+c) into the quadratic formula to find the value for cos(t). You should then be able to solve to find 't', remembering t=120x, so you can then find what x equals
thanks mate I thought trig equations din;t work like normal quadratics...
11. I cant remember what its called when its 0 :L if you imagine how y=tanx cuts through 0, if a graph does that shape but actually goes to a gradient of 0 in the middle, then d^2y/dx^2=0 sorry not a great explaination i know
12. No probs. If my memory serves me well, it's a point of inflexion. I miss this stuff, :-)
13. solved it apparently the solution is ymax = 7

is that obvious from the question?

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
2. this can't be left blank
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
1. Oops, you need to agree to our Ts&Cs to register

Updated: March 22, 2012
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

### iGCSE English Language

Here's the unofficial markscheme

Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read here first

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams