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x+y+z=0 x+y-2z=0 y+z-2x=0 z+x-2y=0
sinA=sinB cosA=sinB sinA=cosB ∠A.+∠B.=90°
f(-1)<f(9)<f(13) f(13)<f(9)<f(-1) f(9)<f(-1)<f(13) f(13)<f(-1)<f(9)
sinA=sinB cosA=cosB sinA=cosB sin(A+B)=sinC
)x+y<0 ( )x+y>0 ( )x-y<0 ( )x-y>0
a+b>0 a-b>0 ab>0
x+y+z=0 x+y﹣2z=0 y+z﹣2x=0 z+x﹣2y=0
a+b>0 a﹣b>0 ab>0
a+b>0 a﹣b>0 ab>0
a+b>0 a-b>0 ab>0
tanA=cotB tanAcotB=1 (sinA)+(cosA)=1 ( )(sinA)+(sinB)=1