f"(x) < 0, which means the function is concave down at all values of x.
For any such function, within the domain of a ≤ x ≤ b, the secant line S(x) is below the curve of f(x), and the tangent line T(x) is above the curve of f(x).
If 1/2 an hour only used up 1/9 of her battery, 9, 1/2 would give us 9/2, which if you simplified it gets you 8 1/2 hours. Hope this helps! Have a great day!
