急!：几道简单高中数学题（三角函数）——15分钟之内!加分!

cos(225度) = -根号2 /2
sin（25π/6） = 1/2
sin(-17π/3) = 根号3 /2
tan(-32π/3) = 根号3
sin85°= cos5°
cos（2/5 π）= sin（1/10 π）
tan（π/3）= cot(π/6)
sin a - cos a=1/5
1-2sinacosa=1/25
24/25=2sinacosa
12/25=sinacosa
sin^3 a - cos^3 a
= (sina-cosa)((sina-cosa)^2+3sinacosa)
= (1/5)((1/5)^2+3*12/25)
= 37/125
sin^4 a - cos^4 a
= (sin^2 a + cos^2 a)(sin^2 a - cos^2 a)
= 1*(sin a + cos a)(sin a - cos a)
= 正负根号(1+2cosasina)*1/5
= 正负根号(49/25)*1/5
= 正负7/5*1/5
= 正负7/25

1.cos225°sin（25π/6）=cos225°sin750°=-cos45°sin30°=-(根号2)/2乘以1/2=-(根号2)/4