What is the common difference of four terms in an AP such that the ratio of the product of the first and fourth terms to that of the second and third is 2:3 and the sum of all four terms is 20 ?
(a) 3
(b) 1
(c) 4
(d) 2
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Answer: (d)2
Step-by-step explanation:
Let four consecutive terms ; (a - 3d), (a - d) , (a + d), (a + 3d) are in AP.
a/c to question,
sum of all four terms is 20
or, (a - 3d) + (a - d) + (a + d) + (a + 3d) = 20
or, 4a = 20
or, a = 5......(1)
again a/c to question,
the ratio of the product of first and fourth terms to that of second term and third is 2:3.
so, (a - 3d)(a + 3d)/(a - d)(a + d) = 2/3
or, (a² - 9d²)/(a² - d²) = 2/3
or, 3a² - 27d² = 2a² - 2d²
or, a² = 25d²
from equation (1),
(5)² = 25d²
or, d² = 1 => d = ±1
here common difference = (a - d) - (a - 3d) = (a + d) - (a - d) = (a + 3d) - (a + d) = 2d
so, common difference = 2(±1) = ±2
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