$\begin{array}{11} (a)\; \text{multiple by 1, multiply by 2, multiple by 3 and so on} \\ (b)\; \text{add zero, multiply by 2, add 2 multiply by 3 and so on} \\ (c)\; \text{multiply 2 add 2 multiply 1 add 3} \\ (d)\; \text{add 0, add 2 multiply 1 add 3} \\ (d)\; \text{add 0, add 2, add 4, add 6 and so on} \end{array}$
