Core idea
AP nth term a+(n−1)d; GP nth term ar^(n−1). For a finite GP, sum=a(r^n−1)/(r−1), r≠1.
Worked check
AP 3,7,11,… has 10th term 39.
Practice
- Show each step and state any domain or assumption.
- Check the result by substitution, estimation or a second method.