What is the vertex f(x)= 5x^2+20x-16

F(x) = 5x² + 20x - 16

y = 5(x² + 4x) - 16
y + 16 = 5(x² + 4x + __)

perfect square:
x² ⇒ x * x
4x ⇒ 2*2x
2² = 4

y + 16 + 4 = 5(x² + 4x + 4)
y + 20  = 5(x+2)²

(x+2)² ⇒ (x+2)(x+2) = x(x+2) + 2(x+2) = x² + 2x + 2x + 4 = x² + 4x + 4

y = 5(x+2)² - 20

The correct answer is c

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