r² + 648 = 700 + 20 + 9
r² = 648 = 729
r² = 729 - 648
r² = 81
r² -81 = 0
(r-9)(r+9) = 0
r -9 = 0 => r = 9
r + 9 = 0 => r = -9
r × r + 648 = 7 × 100 + 2 × 10 + 3 × 3
r × r + 648 = 700 + 20 + 9
r × r + 648 = 729
r × r = 729 - 648
r × r = 81 ⇒ r = 9 ; 9 x 9 = 81
Raspuns: r = 9
