Question

If 5x -4y =15 and xy =3 fine the value of 25x² 16y²

Answer 1

$5x - 4y = 15 \\ xy = 3 \\ 25 {x}^{2} - 16 {y}^{2} \\ = ( {5x)}^{2} + ( {4y)}^{2} - 40xy + 40xy \\ = ( {5x - 4y)}^{2} + 40xy \\ = ( {15)}^{2} + 40 \times 3 \\ = 225 + 120 \\ = 345$