x2+y2=2020√2. . . (i) and x2−y2=2020‌‌... (ii) Adding Eqs. (i) and (ii), we get ∴‌‌x2=1010(√2+1),y2=1010(√2−1) ∴‌‌x2=1010(√2+1),y2=1010(√2−1) ∴‌‌x2y2=(1010)2{(√2)2−(l)2} =(1010)2 ∴‌‌xy=±1010 ∵‌‌x2+y2=2020√2,x2−y2=2020 ⇒‌‌2x+2yy′=0 and 2x−2yy′=0 ⇒‌‌y′=‌