Giải pt
[TEX]\ sin^2x + sin^2y +sin^2(x+y) =\frac{9}{4} [/TEX]
[TEX]VT= P= \frac{1-cos2x}{2}+ \frac{1-cos2y}{2}+sin^2(x+y) [/TEX]
[TEX]= 1- cos(x+y).cos(x-y)+1-cos^2(x+y) [/TEX]
[TEX]\Leftrightarrow cos^2(x+y)+ cos(x+y).cos(x-y) +P-2=0 [/TEX] có nghiệm .
[TEX]\Leftrightarrow \Delta=cos^2(x-y)- 4P+8\ge 0[/TEX]
[TEX]\Rightarrow P\le \frac{9}{4}[/TEX]
Quay lại bài toán :
[TEX]sin^2x + sin^2y +sin^2(x+y) =\frac{9}{4} [/TEX]
[TEX]\Leftrightarrow \left{cos(x-y)=\pm 1\\cos(x+y)=\pm \frac{1}{2}[/TEX]
[TEX]OK^n[/TEX]