TMUA 2021 · Paper 1 · Question 16

Consider the expansion of

(a + b x)^{n}

The third term, in ascending powers of $x$ , is $105 x^{2}$

The fourth term, in ascending powers of $x$ , is $210 x^{3}$

The fourth term, in descending powers of $x$ , is $210 x^{3}$

Find the value of $\frac{a ^{2}}{b}$

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