1）2cos的平方a-1/1-2sin的平方a
=cos2a/cos2a
=1
2）根号1-sin的平方100°.
=√(1-sin100°)(1+sin100°)
=√(1-2sin50°cos50°)(1+2sin50°cos50°)
=√(sin50°-cos50°)^2(sin50°+cos50°)^2
=(sin50°-cos50°)(sin50°+cos50°)
=sin^2 50°-cos^2 50°
=-cos100°
=cos80°