(a) If the coefficient of \(x^{2}\) and \(x^{3}\) in the expansion of \((p + qx)^{7}\) are equal, express q in terms of p.
(b) A man makes a weekly contribution into a fund. In the first week, he paid N180.00, second week N260.00, third week N340.00 and so on. How much would he have contributed in 16 weeks?
(a) Equal coefficients of \(x^{2}\) and \(x^{3}\) in \((p+qx)^{7}\).
The general term is \({}^{7}C_r\,p^{7-r}(qx)^{r}\).
\[\text{Coeff of }x^{2}:\ {}^{7}C_2\,p^{5}q^{2}=21p^{5}q^{2}\]\[\text{Coeff of }x^{3}:\ {}^{7}C_3\,p^{4}q^{3}=35p^{4}q^{3}\]
Set them equal:
\[21p^{5}q^{2}=35p^{4}q^{3}\;\Rightarrow\;21p=35q\;\Rightarrow\;q=\frac{3p}{5}\]
(b) Weekly contributions 180, 260, 340, ...
This is an A.P. with \(a=180\), common difference \(d=80\), and \(n=16\).
\[S_{16}=\frac{n}{2}\big[2a+(n-1)d\big]=\frac{16}{2}\big[2(180)+15(80)\big]\]\[=8\,[360+1200]=8(1560)=12480\]
He would have contributed \(\text{N}12{,}480.00\) in 16 weeks.