You are Here: Home >< Maths

# Rcosalpha, how do I know to use double angle formula and not the factor formula? watch

1. For example:

Solve between 0 and 360, 8cosθ + 15sinθ = 10

How do I know to use the double angle formula and not the factor formula?

Also, I've seen the solutions so I know it's the double angle formula. However, none of the formulas (eg. Cos(A+-B) = CosACosB -+ sinAsinB) have just a cos and a sin in them, they have a coscos and a sinsin (just like the one I just said as an example), so how do you know which of the double angle formulas to use?

Thanks
2. (Original post by vector12)
For example:

Solve between 0 and 360, 8cosθ + 15sinθ = 10

How do I know to use the double angle formula and not the factor formula?

Also, I've seen the solutions so I know it's the double angle formula. However, none of the formulas (eg. Cos(A+-B) = CosACosB -+ sinAsinB) have just a cos and a sin in them, they have a coscos and a sinsin (just like the one I just said as an example), so how do you know which of the double angle formulas to use?

Thanks
I would go with the latter, actually. Consider

.

You can compare the terms,

and

.

Are you familiar with determining and here?
3. (Original post by _gcx)
I would go with the latter, actually. Consider

.

You can compare the terms,

and

.

Are you familiar with determining and here?
What do you mean by you would go with the latter?

Yes, I do know how to compare terms to determine R and alpha. Looking at the mark scheme, I don't see how your way would work since tan alpha = Rsin(a)/Rcos(a) which would be 8/15, like the mark scheme, rather than your way which would give tan alpha to be 15/8, which would mess up the value for alpha wouldn't it?
4. (Original post by vector12)
What do you mean by you would go with the latter?

Yes, I do know how to compare terms to determine R and alpha. Looking at the mark scheme, I don't see how your way would work since tan alpha = Rsin(a)/Rcos(a) which would be 8/15, like the mark scheme, rather than your way which would give tan alpha to be 15/8, which would mess up the value for alpha wouldn't it?
How did you get from my post? Are you sure you didn't misread for and vice versa?

### Related university courses

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: October 14, 2017
Today on TSR

### Summer Bucket List is Back!

Start yours and you could win £150!

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