Find a point on the curve y = 3x2 – 9x + 8 at which the tangents are equally inclined with the axes.
Given:
The curve is y = 3x2 – 9x + 8
Differentiating the above w.r.t x
⇒ y = 3x2 – 9x + 8
⇒ = 2
3x2 – 1 – 9 + 0
⇒ = 6x – 9 ...(1)
Since, the tangent are equally inclined with axes
i.e, or
= The Slope of the tangent = tan
⇒ = tan(
) or tan(
)
⇒ = 1or – 1 ...(2)
tan(
) = 1
From (1) & (2),we get,
⇒ 6x – 9 = 1 0r 6x – 9 = – 1
⇒ 6x = 10 0r 6x = 8
⇒ x = or x =
⇒ x = or x =
Substituting x = or x =
in y = 3x2 – 9x + 8,we get,
When x =
⇒ y = 3()2 – 9(
) + 8
⇒ y = 3() – (
) + 8
⇒ y = () – (
) + 8
taking LCM = 9
⇒ y = ()
⇒ y = ()
⇒ y = ()
⇒ y = ()
when x =
⇒ y = 3()2 – 9(
) + 8
⇒ y = 3() – (
) + 8
⇒ y = () – (
) + 8
taking LCM = 9
⇒ y = ()
⇒ y = ()
⇒ y = ()
⇒ y = ()
Thus, the required point is (,
) & (
,
)