what is the twentieth term of the arithmetic sequence 21, 18, 15, 12, ?

78

-39

-36

1

The 20th term of the arithmetic sequence will be -36.

The nth teem of an arithmetic sequence is calculated by using the formula: a + (n - 1) d.

The 20th term will be:

= a + (n - 1) d.

= a + (20 - 1)d

= a + 19d

where,

a = first term = 21

d = common difference = 2nd term - 1st term = 18 - 21 = -3

Therefore, 20th term will be:

= a + 19d

= 21 + 19(-3)

= 21 - 57

= -36

Therefore, the 20th term is -36

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-36.

Step-by-step explanation:

First term a1 = 21.

Common difference d = 18 - 21 = -3   (15-18 = -3)

nth term an = a1 + (n - 1)d

So 20th term = 21 + (20-1) -3

= 21 - 3 * 19

= 21 - 57

= -36.

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