Question
Factorise: $1+\frac{\mathrm{q}^3}{125}$

Answer

$\begin{aligned} & \text { } 1+\frac{\mathrm{q}^3}{125} \\ & =1^3+\left(\frac{\mathrm{q}}{5}\right)^3 \\ & \text { Here, } a=1 \text { and } b=\frac{q}{5} \\ & \therefore \quad 1+\frac{\mathrm{q}^3}{125}=\left(1+\frac{\mathrm{q}}{5}\right)\left[1^2-1\left(\frac{\mathrm{q}}{5}\right)+\left(\frac{\mathrm{q}}{5}\right)^2\right] \\ & \ldots\left[\because a^3+b^3=(a+b)\left(a^2-a b+b^2\right)\right] \\ & =\left(1+\frac{q}{5}\right)\left(1-\frac{q}{5}+\frac{q^2}{25}\right) \\ & \end{aligned}$

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