Bonsoir
x + x² = 15.75
x² + x - 15 .75 = 0
Δ = 1 - 4 ( 1 * - 15.75) = 64 = 8²
x 1 = ( - 1 - 8 ) / 2 = - 9/2 = - 4.5
x 2 = ( - 1 + 8 ) /2 = 7/2 = 3.5
3.5 + 3.5² = 15.75
- 4.5 + ( - 4.5 )² = 15.75
( x + 1/2 )² - 1 /4
= x² + 2 x /2 + 1 /4 - 1/4
= x² + x
