Question

Two terms of an arithmetic sequence are a12=70 and a30=124. Write an explicit rule for the nth term.

Answer 1

Answer:

The explicit formula is Tn = 34 + 3n

Step-by-step explanation:

The nth term of an arithmetic sequence can be represented using the formula

Tn = a + (n-1)d

where a is the first term , n is the number of terms and d is the common difference

For the 12 term(a12), n = 12

70 = a + (12-1)d

70 = a + 11d •••••••(i)

For the 30th term, n = 30

124 = a + (30-1)d

124 = a + 29d •••••••(ii)

Now, directly subtract equation i from ii, we have;

124-70 = (a-a) + (29d-11d)

54 = 18d

d = 54/18

d = 3

To get a , we substitute the value of d in any of the equations

let’s use the first

70 = a + 11d

substituting d = 3

70 = a + 11(3)

70 = a + 33

a = 70-33

a = 37

Thus the explicit formula for the nth term will be

Tn = a + (n-1)d

where a = 37 and d = 3

Tn = 37 + (n-1)3

Tn = 37 + 3n -3

Tn = 34 + 3n