Simplify the expression:   sin(2cos^(-1)6x)  Thoughts?  =)

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beckden's profile pic

beckden | High School Teacher | (Level 1) Educator

Posted on


Using the double angle formula

`sin(2theta)=2cos(theta)sin(theta) `

we get


Now `sin(theta)=sqrt(1-(cos(theta))^2)` by the pythagorean theorem


And using `cos(cos^(-1)(a))=a` by definition we get


We get by simplifying

`sin(2cos^(-1)(6x))=12xsqrt(1-36x^2)` which is our answer

giorgiana1976's profile pic

giorgiana1976 | College Teacher | (Level 3) Valedictorian

Posted on

Let cos^-1(6x) = arccos 6x

sin(2cos^(-1)(6x)) = sin(2arccos (6x))

We'll use the following identity:

sin (2alpha) = 2sin (alpha)*cos(alpha)

We'll put alpha = arccos 6x

sin(2arccos (6x)) = 2sin (arccos 6x)*cos(arccos 6x)

But sin(arccos alpha) = sqrt(1 - alpha^2) and cos(arccos alpha) = alpha

sin (arccos 6x) = sqrt(1 - 36x^2)

cos (arccos 6x) = 6x

sin(2arccos (6x)) = 2*6x*sqrt(1 - 36x^2)

sin(2arccos (6x)) = 12xsqrt(1 - 36x^2)

Therefore, the requested value is sin(2arccos (6x)) = 12xsqrt(1 - 36x^2).

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