1.通分得:
(sin80-根号3*sin10)/sin10sin80
=2*(sin80*1/2-sin10*根号3/2)/sin10sin80
=2*(sin80cos60-cos80sin60)/sin10cos10
=2*sin20/sin10cos10
=4*sin10cos10/sin10cos10
=4
2.因为sinacosa=1/3,则1/2*sin2a=1/3,即sin2a=2/3
因为a在(0,pi),且sinacosa=1/3>0,故,a在(0,pi/2)
因为sin2a=2/3<sin45,故,a又在(0,pi/4),2a在(0,pi/2)
所以cos2a=根号(1-4/9)=根号5/3
3.cos2a=1-2(sina)^2=2(cosa)^2-1
原式=(2sinacosa-2(sina)^2)(2sinacosa+2(sina)^2)/2sin2a*cos2a
=4(sina)^2*[(cosa)^2-(sina)^2]/2sin2a*cos2a
=2(sina)^2/2sinacosa
=tana
4.
因为sina-cosa=1/5,两边平方得1-2sinacosa=1/25
即2sinacosa=sin2a=24/25>0,故a在(0,pi/2)
又sina-cosa=1/5>0,sina>cosa,则a在(pi/4,pi/2)
故cos2a<0,sin2a=24/25,cos2a=-7/25,
因此sin(2a-pi/4)=根号2/2*(sin2a-cos2a)=根号2/2*(24/25+7/25)=31又根号2/50
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