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ǰλãҳ>>Ϊʲô-1<=cos(A-B)<=1?>>

Ϊʲô-1<=cos(A-B)<=1?

cosa=1/7sina=43/7 tana=sina/cosa = 43 tan(-a)=-tana=- 43 cosb=cos(a-b-a)=cos(a-b)cosa+sin(a-b)sina 0

⣺(a+b) dot (a-2b)=0|a|^2-2|b|^2-a dot b=0 1-2|b|^2-|a|*|b|*cos=0ԣcos=(1-2|b|^2)/|b| Ϊ[0,]cos[-1,1] ԣ-1(1-2|b|^2)/|b|1ȼڣ(1-2|b|^2)/|b|1(1-2|b|^2)/|b|-1 2|b|^2+|b|-10...

ߦ/2

ʲôҲ˵ ֻ˵Ϊ Ϊab=|a|*|b|*cos=1 cos=1/(|a|*|b|)>0 ͶӰҲûʲôλ֮˵ abͶӰ|a|cos=ab/|b|=1/|b| baͶӰ|b|cos=ab/|a|=1/|a|

0

|a|=|b|=1 |a+b|=3 |a+b|^2=3 a^2+b^2+2ab*cos=3 1+1+2ab*cos=3 2ab*cos=1 |a-b|^2=a^2+b^2-2ab*cos=1+1-1=1 |a-b|=1 |3a-2b|^2=9a^2+4b^2-12ab*cos=9+4-12=1 |3a-2b|=1 עÿĸһͷ cosa,bļнǣ

http://hi.baidu.com/522597089/album/item/27a8be051d950a7b2b94c5720ad162d9f3d3c903.html#

cosa=1/7 0

ǺͲʽ 0

; y=cosasinx12a32 =1-sin2xasinx12a32 =-(sinx-a/2)2+a2/4+1-1/2a-3/2 l -1 =

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