Question

Please help me answer my question. I really need help with understanding this.

If
`sinx=(3a)/(2)` and 0 is less than x is less than
`(\pi )/(2)`, express
`(x)/(4)-sin2x` as a function of a.

Answer 1

If sin(x) = 3a/2 and 0 ≤ x ≤ π/2, then x = arcsin(3a/2). The condition on x here is useful because it makes the sin and arcsin functions exacts inverses of one another: if y = sin(x), then arcsin(y) = x.

Recall the double angle identity for sine:

sin(2x) = 2 sin(x) cos(x)

Also because 0 ≤ x ≤ π/2, we know both sin(x) > 0 and cos(x) > 0. So from the Pythagorean identity, it follows that

sin²(x) + cos²(x) = 1

==> cos(x) = √(1 - sin²(x)) = √(1 - 9a ²/4) = 1/2 √(4 - 9a ²)

Then we have

x/4 - sin(2x) = x/4 - 2 sin(x) cos(x)

… = 1/4 arcsin(3a/2) - 2 (3a/2) (1/2 √(4 - 9a ²))

… = 1/4 arcsin(3a/2) - 3a/2 √(4 - 9a ²)