Answer:
a. r1 = 3/2; d = 1/2
Explanation:
You can use your number sense to solve this.
Terms r6 and r8 are two terms apart, so their difference will be 2 times the common difference:
d = (r8 -r6)/2 = (5 -4)/2 = 1/2 . . . . . matches choices A and D
The given terms are even terms. Odd terms will be 'something and 1/2'. The first term of the sequence (n=1) is an odd term, so cannot be 1 (choice D). The only viable answer choice is A.
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If you want to solve for r1, use the equation for the general term and put in the numbers for either r6 or r8. (We'll use r6.)
rn = r1 +d(n -1)
r6 = r1 +(1/2)(6 -1) = 4
r1 = 4 -5/2 = 3/2 . . . . matches choice A