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Let a be the first of the three number and r the common ratio. Then

a*ar*ar2 = a3r3 = 216 = 63 so that ar = 6

Also, a + ar + ar2 = 21

Substitute ar = 6 to give 6/r + 6 + 6r = 21 or 6 + 6r + 6r2 = 21r

So that 6r2 - 15r + 6 = 0 or 2r2 - 5r + 2 = 0

2r2 - 4r - r + 2 = 0 or 2r*(r-2) - 1(r-2) = 0 or (2r-1)*(r-2) = 0 so r = 0.5 or r = 2

r = 0.5 gives the three numbers as being 12, 6, 3

and r = 2 gives 3, 6, 12.

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Q: Three numbers in GP sum up to 21. Their product is 216. Find the numbers.?
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