x²+y²=58
xy=21
x²+y²=58
x=21/y
(21/y)²+y²=58
441/y²+y²=58
441+y⁴=58y²
y⁴-58y²+441=0
t=y²
t²-58t+441=0
Δ=(-58)²-4*1*441
Δ=3364-1764
Δ=1600
√Δ=40
t₁=(-(-58)-40)/(2*1)
t₁=18/2
t₁=9
t₂=(-(-58)+40)/(2*1)
t₂=98/2
t₂=49
9=y²
y=-3 ∨ y=3
49=y²
y=-7 ∨ y=7
x=21/-3 ∨ x=21/3
x=-7 ∨ x=7
x=21/-7 ∨ x=21/7
x=-3 ∨ x=3
(-7,-3),(7,3),(-3,-7),(3,7)
