## L8 Continuity and Differentiability

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निगाहों में मंजिल थी,
गिरे और गिरकर संभलते रहे,
हवाओं ने बहुत कोशिश की,
मगर चिराग आँधियों में भी जलते रहे।

# Lecture - 8 Chapter 5 Continuity and Differentiability

In this lecture, I am discussing remaining questions from NCERT Exercise 5.5 which are based on logarithmic differentiation.

Questions discussed in this lecture:

 NCERT EXERCISE 5.5 (Logarithmic Differentiation)

Find $$\frac{dy}{dx}$$ of the functions given in Exercises 12 to 15.

Question 12. $$x^y + y^x =1$$

Question 13. $$y^x = x^y$$

Question 14. $$(\cos x)^y = (\cos y)^x$$

Question 15. $$xy = e^{(x-y)}$$

Question 16. Find the derivative of the function given by $$f(x) = (1+x)(1+x^2)(1+x^4)(1+x^8)$$ and hence find f′(1).

Question 17. Differentiate $$(x^2-5x+8)(x^3+7x+9)$$ in three ways mentioned below:
(i) by using product rule
(ii) by expanding the product to obtain a single polynomial.
(iii) by logarithmic differentiation.
Do they all give the same answer?

Question 18. If u, v and w are functions of x, then show that $$\frac{d}{dx}(u.v.w) = \frac{du}{dx}.v.w + u.\frac{dv}{dx}.w + u.v.\frac{dw}{dx}$$ in two ways – first by repeated application of product rule, second by logarithmic differentiation.

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