因为 sina^2+cosa^2=1所以 1-sin2β^2=cos2β^2原式即 cos2β^2=-cos2β得 cos2β=-1或0当 cos2β=-1时得 2β=π ==> β=π/2当 cos2β=0时得 2β=π/2 或 3π/2 ==> β=π/4 或 3π/4综上 β= π/2 或 π/4 或 3π/4
易知,cos2b≤0.===>π/2≤2b≤3π/2.===>π/4≤b≤3π/4