9514 1404 393
Answer:
D: 21
Step-by-step explanation:
The first term is (a1). The third term is (a1)r². The product of these terms is ...
(a1)(a1)r² = ((a1)r)², the square of the second term.
The missing term is √(7·63) = √441 = 21.
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Additional comment
In consideration of the above, you can also see that the common ratio can be found from ...
a3/a1 = ((a1)r²)/(a1) = r² = 63/7 = 9
Then the common ratio is r = √9 = 3, and the second term is 7·3 = 21.
