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CLASS_11_MATHS_SOLUTIONS_NCERT

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Class XI Chapter 13 – Limits and Derivatives Maths<br />

______________________________________________________________________________<br />

Question 3:<br />

Find the derivative of x at x = 1.<br />

Solution 3:<br />

Let f(x) = x. Accordingly,<br />

f 1h f 1<br />

f ' 1<br />

lim<br />

h0<br />

h<br />

1h<br />

1<br />

lim<br />

h0<br />

h<br />

h<br />

lim<br />

h0<br />

h<br />

lim 1<br />

h0<br />

<br />

= 1<br />

Thus, the derivative of x at x = 1 is 1.<br />

Question 4:<br />

Find the derivative of the following functions from first principle.<br />

3<br />

x – 27 (ii) (x – 1) (x – 2)<br />

(i)<br />

1 x1<br />

(ii) (iv)<br />

2<br />

x x1<br />

Solution 4:<br />

3<br />

(i) Let f(x) = x – 27. Accordingly, from the first principle,<br />

f x h<br />

f x<br />

f ' x<br />

lim<br />

h0<br />

h<br />

3<br />

<br />

3<br />

x h 27 x<br />

27<br />

lim<br />

<br />

h0<br />

h<br />

3 3 2 2 3<br />

x h 3x h3xh x<br />

lim<br />

h0<br />

h<br />

3 2 2<br />

h 3x h3xh<br />

lim<br />

h0<br />

h<br />

3 2 2<br />

lim h 3x h3xh<br />

h0<br />

<br />

<br />

2 2<br />

03x<br />

0 3x<br />

(ii) Let f(x) = (x – 1) (x – 2). Accordingly, from the first principle,<br />

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