Mathematics for IGCSE & O level - Algebraic Manipulation (Section 5 - No. 9)
When completing the square for the expression x² - 4x + 5, the number to add and subtract inside the parentheses is:
1
2
4
5
Explication
You take half of the coefficient of the x term (-4), square it (-2² = 4), and that's the value to add and subtract.