[shanban]

I was looking through a solution someone on here posted to a q and I'm not sure I follow...


Can someone pls explain the highlighted step and how you'd go on from there?

Thank you!

(Original post by Horizontal 8)
STEP I question 3






But
Hence no solutions arise from this.

Also,

So we use to find the other angles.

Cosx is -ve for when considering 0<x<2pi

Therefore the angles that we want are:





And this is equal to

As required.

[Ree69]

Notice that

[shanban]

hmm I still don't understand why you wouldn't just use the cosx instead of cos(-x) to work through the remainder of the question???

[z0tx]

(Original post by shanban)
hmm I still don't understand why you wouldn't just use the cosx instead of cos(-x) to work through the remainder of the question???

I think it is because the interval you are dealing with in the equation is . Since , you rule it out and use the equivalent positive value, which is

[shanban]

(Original post by z0tx)
I think it is because the interval you are dealing with in the equation is . The absolute value gives two possibilities. Since , you rule it out and use the other (positive) value, which is

oh of course! got it, thank you