2.(5/ 12π divided by 2π)*2π*30cm

5/ 12π divided by 2π is the percentage of arc length to the circumference, and 2π*30cm is the circumference.

3. Draw the rectangular coordinate system xoy, 3, 4, 5; 5 is the op length.

So Sina =-4/5, COSA =-3/5.

4.3/2 < π/2, and sinα is less than 0, so the quadrant of A is three or four, so COSA =+- 12/ 13.

5.tanα is less than 0, so the quadrant where A is located is in two or four.

And α∈(π/2, π), so A is in the second quadrant, so sina/cosa =-2 and sina +cosa = 1, SINA is positive and cosa is negative.

6.sin^2 α-sin^2α* sin^2 β=sin^2 α*( 1-sin^2 β)=sin^2 α*cos^2β

sin^2 α*cos^2 β+cos^2α* cos^2 β=cos^2 β*(sin^2 α+cos^2α)=cos^2β

Cos 2β Re+Sin 2β = 1