Question

The 10th and 15th terms of an AP are _5 and _7 1/2, respectively.what is the sum of the first 20 terms.

Answer 1

_ is not the same as -

d = -2.5/5 = -1/2
so a = -5 + 9/2 = -1/2
S20 = 20/2 (2a+19d) = ____