Mathematics

Mathematics

Problem

In a geometric progression, the 3rd term is 4 and the 5th term is 16. Which of the following gives the possible value(s) of the common ratio? a) 2 only2\text{ only} b) 2 or −22 \text{ or } -2 c) 12\dfrac12 d) 4 only4\text{ only}
Intuitive explanation

A geometric progression grows (or shrinks) by multiplying by the same factor each time—that's the common ratio rr. The key insight is that you can write any term using the first term and rr, then use your two known terms to set up equations and solve for rr. Since squaring is involved, you'll likely get two solutions, both of which are valid because a geometric sequence can have a negative common ratio and still produce positive terms.

Intuitive explanation

A geometric progression grows (or shrinks) by multiplying by the same factor each time—that's the common ratio rr. The key insight is that you can write any term using the first term and rr, then use your two known terms to set up equations and solve for rr. Since squaring is involved, you'll likely get two solutions, both of which are valid because a geometric sequence can have a negative common ratio and still produce positive terms.

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