EXAMPLE 6
Find an equation of the tangent line to the parabola y = x^2 - 6x + 8 at the point (1, 3).
SOLUTION
From the previous example, we know the derivative of f(x) = x^2 - 6x + 8 at the number a is f'(a) = 2a - 6. Therefore, the slope of the tangent line at (1, 3) is f'(1) = 2(1) - 6 = -4.
Thus, an equation of the tangent line, shown in the figure, is y - 3 = -4(x - 1) or y = -4x + 7.