Now xy+4x−5y−20=0 (x−5)(y+4)=0⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅(I) And xy−5x+4y−20=0 (x+4)(y−5)=0⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅(II) The equation of diagonal AC is, y+4=x+4 x−y=0 The equation of diagonal BD is, y+4=−x+5 x+y−1=0 Combined equation of diagonal are, (x−y)(x+y−1)=0 x2−y2−x+y=0 Hence, k=1 l=1 Therefore, k+l=1+1 =2