Find the point on the curve y = x3 – 11x + 5 at which the equation of the tangent is y=x–11.
The equation of the given curve is y = x3 − 11x + 5.
The equation of the tangent to the given curve is given as y = x − 11 (which is of the form y = mx + c).
∴ Slope of the tangent = 1
Now, the slope of the tangent to the given curve at the point (x, y) is given by, = 6(x + 1)² (x – 3)² (x – 1)
Then, we have:
f’ (x) = 0 ⇒ x = -1, 3, 1
When x = 2, y = (2)3 − 11 (2) + 5 = 8 − 22 + 5 = −9.
When x = −2, y = (−2)3 − 11 (−2) + 5 = −8 + 22 + 5 = 19.
Hence, the required points are (2, −9) and (−2, 19).
But point (- 2, 19) does not satisfy the equation of tangent.
Therefore, required point is (2, - 9).