NCERT Solutions
Class 12 Maths
Application of Derivatives

Ex.6.3 Q.15
Find the equation of the tangent line to the curve y = x2 − 2x + 7 which is
(a) parallel to the line 2x − y + 9 = 0
(b)perpendicular to the line 5y − 15x = 13
The equation of the given curve is y = x2 − 2x + 7
On differentiating with respect to x, we get:
= 2x - 2
(a) The equation of the line is 2x − y + 9 = 0
⟹ y = 2x + 9
This is of the form y = mx + c
So, the slope of the line = 2
If a tangent is parallel to the line 2x − y + 9 = 0, then the slope of the tangent is equal to the slope of the line.
Therefore, we have:
2 = 2x − 2
⟹ 2x = 4
⟹ x = 2
Now, x = 2,
y = 2(2) − 4 + 7
⟹ y = 4 – 4 + 7
⟹ y = 7
Thus, the equation of the tangent passing through (2, 7) is given by,
y – 7 = 2(x - 2)
⟹ y – 7 = 2x – 4
⟹ y – 2x – 3 = 0
Hence, the equation of the tangent line to the given curve (which is parallel to line 2x − y + 9 = 0) is y – 2x – 3 = 0
(b) The equation of the line is 5y − 15x = 13
5y = 15x + 13
⟹ y = 3x +
This is of the form y = mx + c
So, the slope of the line = 3
If a tangent is perpendicular to the line 5y − 15x = 13, then the slope of the tangent is
=
⟹ 2x – 2 =
⟹ 2x = + 2
⟹ 2x =
⟹ x =
Now, at x =
y = ( )2 – 2(
) + 7
⟹ y = –
+ 7
⟹ y = 25 – 60 + 25236
⟹ y =
Thus, the equation of the tangent passing through is given by,
Hence, the equation of the tangent line to the given curve (which is perpendicular to line 5y – 15x = 13) is 36y + 12x – 217 = 0