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Explanation:
Let's complete the square
y = x^2 + 8x
y + 16 = x^2 + 8x + 16 .... adding 16 to both sides
y + 16 = (x+4)^2
y = 1(x+4)^2 - 16
We added 16 to both sides to allow the right hand side to factor into a perfect square.
The last line shown above is in the form y = a(x - h)^2 + k where
a = 1
h = -4
k = -16
Note the positive 'a' value indicates we have a minimum.
The vertex is (h,k) = (-4, -16)
The lowest y value possible is at the vertex.
So the smallest y value is y = -16.
This means the smallest possible value of x^2+8x is -16.
