Using the addition formula for sin(x+y), find sin(3x) in terms of sin(x) and hence show that sin(10) is a root of the equation 8x^3 - 6x + 1

First we state the formula for sin(x+y)

sin(x+y) = sin(x)cos(y) + cos(x)sin(y)

Letting y = 2x

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

Now sin(2x) = 2sin(x)cos(x) and cos(2x) = 1 - 2sin^2(x), substitute these into the formula gives us

sin(3x) = sin(x)(1-2sin^2(x)) + cos(x)(2sin(x)cos(x))

sin(3x) = sin(x) - 2sin^3(x) + 2sin(x)cos^2(x)

Now cos^2(x) = 1 - sin^2(x)

sin(3x) = sin(x) - 2sin^3(x) + 2sin(x)(1-sin^2(x))

sin(3x) = sin(x) - 2sin^3(x) + 2sin(x) - 2sin^3(x)

sin(3x) = 3sin(x) - 4sin^3(x)

Now letting x = 10, we get

sin(30) = 3sin(10) - 4sin^3(10)

Rearranging and evaluation sin(30) = 1/2

8sin^3(10) - 6sin(10) + 1 = 0

Hence sin(10) is a root of the cubic equation