2. Use the angle doubling formula: sin2c = 2 sinc cosc, cos2c = 1-2 (sinc) 2.

3. Use the formula: (COSD) 2 = 1-(SIND) 2.

[proof]

sin3x

=sin(2x+x)

=sin2xcosx+cos2xsinx

=(2sinxcosx)cosx+[ 1-2(sinx)^2]sinx

=2sinx(cosx)^2+sinx-2(sinx)^3

=2sinx[ 1-(sinx)^2]+sinx-2(sinx)^3

=3sinx-4(sinx)^3，

∴4(sinx)^3=3sinx-sin3x,∴(sinx)^3=(3/4)sinx-( 1/4)sin3x。