[phuong trinh luong giac hai an]

T

tuyn

(sinx)^2 + (siny)^2 + sin^2(x+y) = 9/4
\Leftrightarrow(1-cos2x)/2+(1-cos2y)/2+sin^2(x+y)=9/4
\Leftrightarrow-1/2(cos2x+cos2y)+sin^2(x+y)=5/4
\Leftrightarrow -cos(x+y).cos(x-y)+1-cos^2(x+y)=5/4
\Leftrightarrow[cos^2(x+y)+cos(x+y).cos(x-y)+1/4cos^2(x-y)]+1/4[1-cos^2(x-y)]=0
\Leftrightarrow[cos(x+y)+1/2cos(x-y)]^2+1/4sin^2(x-y)=0
 
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