✔ 最佳答案
x² - (p + 2)x + (p + 5) = 0
x² - (p + 2)x = - (p + 5)
x² - 2[(p + 2)/2]x = - (p + 5)
x² - 2[(p + 2)/2]x + [(p + 2)/2]² = - (p + 5) + [(p + 2)/2]²
{x² - [(p + 2)/2]}² = - (p + 5) + [(p + 2)/2]²
Since it is a perfect square, then - (p + 5) + [(p + 2)/2]² = 0
4 {- (p + 5) + [(p + 2)/2]²} = 0
-4(p + 5) + (p + 2)² = 0
-4p - 20 + p² + 4p + 4 = 0
p² - 16 = 0
p² - 4² = 0
(p + 4)(p - 4) = 0
p = -4 or p = 4