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Recover a logarithmic series from a differential identity

Let y=ln⁡(1+sin⁡x)y=\ln(1+\sin x).

(i) Show that eyd2ydx2=1−ey[(dydx)2+1]e^y\dfrac{d^2y}{dx^2}=1-e^y[(\dfrac{dy}{dx})^2+1].

(ii) Find its Maclaurin series through x2x^2.

(iii) Verify it using the standard series for sin⁡x\sin x and ln⁡(1+x)\ln(1+x).

[2007/CJC/I/7]

CJC · Discussion · Question 7

Multiply the first derivative by eʸ before differentiating again.

Read the worked solution