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ǰλãҳ>>cos(-a)=cos aΪʲô>>

cos(-a)=cos aΪʲô

Ϊy=cosxĺͼǹyԳƵģԶxͩx뺯еõĺֵһ

: õյʽ cos(-x)=cosx x=a+У cos(-a-)=cos(+a)

cos(k-)={cos,kΪż -cos,kΪ cos(4-a)==cosa

cos(a-b)=cosacosb+sinasinbǺȱ任Ĺʽ ɣлл

ǰǹXԳƣCOSĽXϵӳ䰣ͼһˡ

Ϊ A+B+C=180 cosC=cos(180-(A+B))=-cos(A+B) sinC=sin(180-(A+B))=sin(A+B) õյʽ cos180-a=-cosa sin180-a=sina

cos(/2+a)=cos(-/2-a)=-cos(-/2-a)=-cos(/2-a)=-sina

cos90+a+cos150-a =2cos120cosa =2-2֮1cosa-30㣩 = -cosa-30㣩 = -cos30-a

ֱΪ, +=90() εֱDZΪa,b бΪc sin =a/c cos =a/c sin =cos Ϊ+=90 sin =cos(90-)

ԡ УA+B=-C cos(A+B) =cos(-C) =-cosC ϣﵽ лл

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