Find the 21st term of the sequence ={15,8,1,-6}

Answer 1

a₁ = 15; d = 8 -15 = - 7 is arithmetic sequence;

a₂₁ = a₁ + 20d = 15 - 7·20 = 15 - 140 = - 125

Answer 2

There is a constant difference between each element of this sequence (7), so this is an arithmetic sequence. The formula for the nth term of an arithmetic sequence is:

a(n) = a(1) + (n-1)d, where d is the constant difference, n is the number of the term you are interested in and a(1) is the first term.