You are Here: Home >< Maths

# C4 Cartesian Parametric Question - why can they suddenly use this? watch

1. Question:

I get part A. It's part B which I don't get. Usually with cartesians, you get one equation in terms of the parametric (eg if you had x=2t and y=4t, you could do t=x/2 and sub that into the second equation), whereas with this one, they haven't done that.

They have just used the fact that cos2t = 1-2sin^2t and used that. How do you know to do that? I could never have spotted it.

Thanks
Attachment 724016724018
Attached Images

2. (Original post by stuart4)
Question:

I get part A. It's part B which I don't get. Usually with cartesians, you get one equation in terms of the parametric (eg if you had x=2t and y=4t, you could do t=x/2 and sub that into the second equation), whereas with this one, they haven't done that.

They have just used the fact that cos2t = 1-2sin^2t and used that. How do you know to do that? I could never have spotted it.

Thanks
Just practise. If you've got sines and cosines, then you look for a way to eliminate them either via an identity, like cos^2+sin^2=1, or in this case since since one is cos2x and the other sin x, it screams double angle formula, and look for a way to use it.

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:
Updated: February 10, 2018
Today on TSR

### Uni league tables

Do they actually matter?

### University open days

• University of Warwick
Sat, 20 Oct '18
• University of Sheffield
Sat, 20 Oct '18
• Edge Hill University
Faculty of Health and Social Care Undergraduate
Sat, 20 Oct '18
Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams