NCERT Solutions
Class 12 Maths
Application of Derivatives

Ex.6.3 Q.14
Find the equations of the tangent and normal to the given curves at the indicated points:
(i) y = x4 − 6x3 + 13x2 − 10x + 5 at (0, 5)
(ii) y = x4 − 6x3 + 13x2 − 10x + 5 at (1, 3)
(iii) y = x3 at (1, 1)
(iv) y = x2 at (0, 0)
(v) x = cos t, y = sin t at t = π/4
(i) The equation of the curve is y = x4 − 6x3 + 13x2 − 10x + 5
On differentiating with respect to x, we get:
= 4x3 − 18x2 + 26x – 10
= 4 (0)3 – 18 (0)2 + 26 (0) – 10
⟹ = -10
Thus, the slope of the tangent at (0, 5) is −10. The equation of the tangent is given as:
y − 5 = − 10(x − 0)
⟹ y − 5 = − 10x
⟹ 10x + y = 5
The slope of the normal at (0, 5) = at (0, 5) = -1/(-10) = 1/10
Therefore, the equation of the normal at (0, 5) is given as:
y − 5 = (x − 0)
⟹ 10y − 50 = x
⟹ x - 10y + 50 = 0
(ii) The equation of the curve is y = x4 − 6x3 + 13x2 − 10x + 5
On differentiating with respect to x, we get:
= 4x3 − 18x2 + 26x – 10
= 4 * 13 – 18 * 12 + 26 * 1 – 10
⟹ = 4 – 18 + 26 – 10
⟹ = 30 – 28
⟹ = 2
Thus, the slope of the tangent at (1, 3) is 2. The equation of the tangent is given as:
y − 3 = 2(x − 1)
⟹ y − 3 = 2x - 2
⟹ y = 2x + 1
The slope of the normal at (1, 3) = at (1, 3) =
Therefore, the equation of the normal at (1, 3) is given as:
y − 3 = (x − 1)
⟹ 2y − 6 = -x + 1
⟹ x + 2y - 7 = 0
(iii) The equation of the curve is y = x3
On differentiating with respect to x, we get:
= 3x2
= 3 (1)2 = 3
Thus, the slope of the tangent at (1, 1) is 3 and the equation of the tangent is given as:
y – 1 = 3(x – 1)
⟹ y – 1 = 3x – 3
⟹ y = 3x - 2
The slope of the normal at (1, 1) = -1/Slope of the tangent at (1, 3) =
Therefore, the equation of the normal at (1, 1) is given as:
y – 1 = (x – 1)
⟹ 3y – 3 = -x + 1
⟹ x + 3y – 4 = 0
(iv) The equation of the curve is y = x2
On differentiating with respect to x, we get:
= 2x
= 2 (0) = 0
Thus, the slope of the tangent at (0, 0) is 0 and the equation of the tangent is given as:
y – 0 = 0(x – 0)
⟹ y = 0
The slope of the normal at (0, 0) = -1/Slope of the tangent at (0, 0) = -1/0
Which is not defined.
Therefore, the equation of the normal at (x0, y0) = (0, 0) is given as:
x = x0 = 0
(v) The equation of the curve is x = cos t, y = sin t
So, and
Now,
Thus, the slope of the tangent at is -1
When ,
Thus, the equation of the tangent at i.e.
is:
The slope of the normal at t at
Thus, the equation of the normal at is
⟹