# Writing A Trig Expression as an Algebraic Expression

## Homework Statement

Write the Trigonometric Expression as an algebraic expression.

cos(2arccos 2x)

## Homework Equations

Probably the inverse properties, I'm not sure.

## The Attempt at a Solution

I know I can rewrite this equation as.

u= arccos 2x

cos(2cos u=2x)

I can also say that the adjacent leg is 2x units long and the hypotenuse is 1 unit long. Then using the pythagorean theorm I can figure the opposite leg to be sqrt(1-4x2)

I'm not sure If this is necessary though can someone point me in the right direction? The 2 in front of the arccos is throwing me off because if that wasn't there I would just use the inverse property and cos(arccos 2x) would equal 2x.

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rock.freak667
Homework Helper
If u=cos-1(2x) then you want to find cos(2u).

cos(2u)=cos2u-sin2u=2cos2u-1 = 1-2sin2u

and cos2u = (cosu)2

I'm not sure I understand why you'd want to find cos(2u)

The answer is supposed to be 8x2-1 and thats the answer listed in the back of the book.

rock.freak667
Homework Helper
I'm not sure I understand why you'd want to find cos(2u)

The answer is supposed to be 8x2-1 and thats the answer listed in the back of the book.
cos(2cos-1(2x))

if you put u = cos-1(2x), wouldn't cos(2cos-1(2x)) become cos(2u)?

Thanks, now I understand.

Once you have simplified it to 2cos2u-1 all you have to do is simplify it with the u in place

cos2(arcsin 2x)=2x

2(2x)2-1

8x2-1

Thanks!