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ǰλãҳ>>cos(-a)=cos aΪʲô>>

cos(-a)=cos aΪʲô

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

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

cosУcos sinУsin tanУtan cotУcot sec(-)=-sec csc(-)=csc

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

cos(90+a)-sina ż䣬ſޡ յ90㣬䣻90+ܦʹյʽʱҪǣ90+Ҫڶ޵Ľǣڶ޽ǵֵǸģcos(90+a)-sina. ǸǴģ

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

ȽֱϵֱϵxOyλԲOa,b-bʹaĿΪOxԲOڵP1ձ߽ԲOڵP2bʼΪOP2ձ߽ԲOڵP3-bʼΪOP1ձ߽ԲOڵP4ʱP1P2P3P4ֱΪ  P110...

cos(a-b)=cosacosb+sinasinbǺȱ任Ĺʽ ɣлл

Ǻ sina=Ա/б cosa=ڱ/б sin2a+cos2a =(Ա/б)^2+(ڱ/б)^2 =(Ա^2+ڱ^2)/б^2 ɹɶ Ա^2+ڱ^2=б^2 sin2a+cos2a=1

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