Arithmetic Progression : 8. Sum of n terms of an A.P or Sn = n2\frac{n}{2}( 2a + (n -1)d) = n2\frac{n}{2}( a + l) 9. t1 + tn = t2 + tn – 1 = t3 + tn – 2 = ……………. 10. Three terms in A.P →\rightarrow a – d, a, a + d ...