Series Expansion via Undetermined Coefficients

Expand the expression $\frac{1 + 2 x}{1 - x - x ^{2}}$ in ascending powers of $x$ as far as $x^{3}$ .

~5 minAnswer checked instantlyundetermined coefficientspower series expansion

H. S. Hall & S. R. Knight, Higher Algebra (1891) · Ch. XXII. Undetermined Coefficients, Examples XXII.b, No. 1

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Checked against the answer in H. S. Hall & S. R. Knight, Higher Algebra (1891).