神马作文网cos(a-β/2)=-1/9,sin(a/2-β）=2/3 且 π/2
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cos(a-β/2)=-1/9,sin(a/2-β）=2/3 且 π/2
π/2＜a＜π,0＜b＜π/2
--->π/4＜a-b/2＜π,∵cos(a-b/2)=-1/9＜0--->π/2＜a-b/2＜π
--->sin(a-b/2)=4√5/9
同时：-π/4＜a/2-b＜π/2,∵sin(a/2-b)＞0--->0＜a/2-b＜π/2
--->cos(a/2-b)=√5/3
cos(a/2+b/2) = cos[(a-b/2)-(a/2-b)]
= cos(a-b/2)cos(a/2-b)+sin(a-b/2)sin(a/2-b)
= (-1/9)(√5/3)+(4√5/9)(2/3)
= 7√5/27
--->π/4＜a-b/2＜π,∵cos(a-b/2)=-1/9＜0--->π/2＜a-b/2＜π
--->sin(a-b/2)=4√5/9
同时：-π/4＜a/2-b＜π/2,∵sin(a/2-b)＞0--->0＜a/2-b＜π/2
--->cos(a/2-b)=√5/3
cos(a/2+b/2) = cos[(a-b/2)-(a/2-b)]
= cos(a-b/2)cos(a/2-b)+sin(a-b/2)sin(a/2-b)
= (-1/9)(√5/3)+(4√5/9)(2/3)
= 7√5/27
cos(a-β/2)=-1/9,sin(a/2-β）=2/3 且 π/2
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