Question

Va rog frumoooosssssss

Answer 1

$A_{dreptunghi}=L*l=x*(x+4)\\x^2+4x=21\\x^2+4x-21=0\\a=1\\b=4\\c=-21\\\Delta=b^2-4ac=16+4*21=100\\x_{1/2}=\frac{-b\pm\sqrt{\Delta} }{2a} =\frac{-4\pm10}{2} \\x_1=\frac{6}{2} =3\\x_2=\frac{-14}{2}=-7 (fals)\\ =>x=3=> l=3, L=7$