Get cos2x cos3x-sin2x sin3x = 0

cos(2x+3x)=0

Cos5x=0 Obviously, when x = 18 is satisfied, the other three options are not satisfied!

2. Let the radius be r and the arc length be l, then 2r+l = 6, 1/2l * r = 2.

The solution is r= 1 and l=4. Or r=2, l=2.

Central angle =l/r=4 or 1.

3 . a 1+a2+a3 = 12 a 1+a3 = 2 a2

3a2= 12，a2=4。

A 1=2, a2=4. So an=2n.

Sn=n(n+ 1)

1/Sn = 1/n- 1/(n+ 1)

So1/s1+1/S2+1/S3 ...+1/s 2010 =1/.

= 1- 1/20 1 1=20 10/20 1 1