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A hint to get started

Verify an implicit logarithm expansion by a second method

It is given that y=ln⁡(2+sin⁡2x)y=\ln(2+\sin2x).

(i) Show that eyd2ydx2+ey(dydx)2=−4sin⁡2xe^y\dfrac{d^2y}{dx^2}+e^y\left(\dfrac{dy}{dx}\right)^2=-4\sin2x. marks

(ii) By further differentiation of the above results, find the Maclaurin series for y, up to and including the term in x3x^3. marks

(iii) Verify that the series for ln⁡(2+sin⁡2x)\ln(2+\sin2x) is the same as the result obtained in part (ii), if the standard series from the List of Formulae (MF27) are used. marks

(iv) Hence deduce the series expansion for ln⁡(2+sin⁡2x)1−x\dfrac{\ln(2+\sin2x)}{\sqrt{1-x}}, up to and including the term in x2x^2. marks

[2022/ACJC/I/7(modified)]

CJC · Lecture · Example 8 · Self-Reading

Differentiate the relation before substituting zero, retain every product-rule contribution, and divide derivative values by the corresponding factorials.

Read the worked solution