NCERT Solutions
Class 11 Maths
Limits and Derivatives

Ex.Misc. Q.1
Find the derivative of the following functions from first principles:
1. −x
2. (-x)-1
3. sin (x + 1)
4. cos (x – )
1.Let f(x) = -x
Now, by first principle, f’(x) = limh->0 [{f (x + h) – f(x)} ÷ h]
= limh->0 [{- (x + h) – (-x)} ÷ h]
= limh->0 [{-x - h + h} ÷ h]
= limh->0 [-h ÷ h]
= limh->0 (-1)
= -1
2. Let f(x) = (-x)-1 = -1 ÷ (-x) = -
Now, by first principle, f’(x) = limh->0 [{f (x + h) – f(x)} ÷ h]
= limh->0 [{-1 ÷ (x + h) – (- )} ÷ h]
= limh->0 [{-1 ÷ (x + h) + } ÷ {h × x (x + h)}]
= limh->0 [(-x + x + h) ÷ {h × x (x + h)}]
= limh->0 [h ÷ {h × x (x + h)}]
= limh->0 [1 ÷ {x (x + h)}]
= 1 ÷ (x × x)
= (1 ÷ x2)
3.Let f(x) = sin (x + 1)
Now, by first principle, f’(x) = limh->0 [{f (x + h) – f(x)} ÷ h]
= limh->0 [{sin (x + h + 1) – sin (x + 1)} ÷ h]
= limh->0 [{2 × cos (x + h + 1 + x + 1) ÷ 2 × sin (x + h + 1 – x - 1) ÷ 2} ÷ h]
= limh->0 [{(2 × cos (2x + h + 2) ÷ 2) × sin ( )} ÷ h]
= limh->0 [{cos (2x + h + 2) ÷ 2} × {sin ( ) ÷ (
)}]
= limh->0 [cos (2x + h + 2) ÷ 2] × limh->0 [sin ( )÷(
)]
= {cos (2x + 0 + 2) ÷ 2} × 1
[Since limx->0 (sin x) ÷ x = 1]
= cos (x + 1)
4. Let f(x) = cos (x – ) Now, by first principle,
f’(x) = limh->0 [{f (x + h) – f(x)} ÷ h]
= limh->0 [{cos (x + h - ) – cos (x -
)} ÷ h]
= limh->0 [{-2 × sin (x + h - + x -
) ÷ 2 × sin (x + h -
- x +
) ÷ 2} ÷ h]
= limh->0 [{-2 × sin (2x + h - ) ÷ {2 × sin (
)} ÷ h}]
= -limh->0 [sin (2x + h - ) ÷ 2 × sin (
) ÷ (
)]
= -limh->0 [sin (2x + h - ) ÷ 2] × limh->0 [sin (
) ÷ (
)]
= {-sin (2x + 0 - ) ÷ 2} × 1
= -sin (x - )