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2x-3y=-5
4x²-9y²=20
2x+3y=?


Răspuns :

[tex]4 x^{2} -9 y^{2} =(2x+3y)(2x-3y) \\ 20=-5(2x+3y) \\ 2x+3y= -\frac{20}{5} =-4[/tex]