2025 EJC Promo Q10

2025 EJC Promo Q10

Junior College 1
9 marks

It is given that \(y=\tan[\ln(1+2x)]\), for \(x>-0.5\).

  1. Show that \((1+2x)\dfrac{dy}{dx}=2(1+y^2)\).[2]
  2. Hence find the first three non-zero terms of the Maclaurin expansion of \(\tan[\ln(1+2x)]\).[4]
  3. It is given that the three terms found in part (b) are equal to the first three terms in the series expansion of \(2x(1+ax)^b\). Find the exact values of the constants \(a\) and \(b\).[3]

Solution:

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Answer:(b) \(2x-2x^2+\dfrac{16}{3}x^3+\cdots\) (c) \(a=\dfrac{13}{3}\), \(b=-\dfrac3{13}\)

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