Priya writes the AP 70, 66, 62, 58, ... Find the sum of its first 30 terms.

$a=70,d=-4,n=30.S=\frac{30}{2}$$[140+29(-4)]=15[140-116]=15(24)=360$. Because $d$ is negative the later terms are negative and cancel most of the total. The value 300 uses 30$d$ instead of 29$d$, 720 forgets the factor $\frac{1}{2}$, and 2100 ignores the $d$ term altogether, as if all 30 terms equalled 70.