The sum of an infinite GP whose common ratio is numerically less than ...
Trying to plug in values we can see that the infinite sum of the GP 16, 8, 4, 2… is 32 and hence the
third term is 4.
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The sum of an infinite GP whose common ratio is numerically less than ...
To find the third term of the infinite geometric progression (GP), we need to determine the common ratio (r) and the first term (a).
Let's denote the first term as a and the common ratio as r.
Given that the sum of the infinite GP is 32, we can use the formula for the sum of an infinite GP:
S = a / (1 - r)
Substituting the given values, we have:
32 = a / (1 - r)
We are also given that the sum of the first two terms is 24. The sum of the first two terms of a GP can be calculated using the formula:
S2 = a + ar
Substituting the given values, we have:
24 = a + ar
From these two equations, we can solve for a and r simultaneously.
Solving for a in the second equation:
24 - ar = a
a = 24 / (1 - r)
Substituting this value of a into the first equation:
32 = (24 / (1 - r)) / (1 - r)
32 = 24 / (1 - r)^2
Multiplying both sides by (1 - r)^2:
(1 - r)^2 * 32 = 24
Expanding and simplifying:
32 - 64r + 32r^2 = 24
32r^2 - 64r + 8 = 0
Dividing through by 8:
4r^2 - 8r + 1 = 0
Using the quadratic formula to solve for r:
r = (-b ± √(b^2 - 4ac)) / (2a)
r = (8 ± √(64 - 16)) / (8)
r = (8 ± √48) / 8
Simplifying:
r = (8 ± 4√3) / 8
r = 1 ± √3/2
Since the common ratio r is numerically less than 1, we take the negative root:
r = 1 - √3/2
Now, substitute this value of r back into the equation for a:
a = 24 / (1 - r)
a = 24 / (1 - (1 - √3/2))
a = 24 / (√3/2)
a = 48 / √3
To find the third term, we use the formula:
T3 = a * r^2
T3 = (48 / √3) * ((1 - √3/2)^2)
T3 = (48 / √3) * (1 - 2√3/2 + 3/4)
T3 = (48 / √3) * (1 - √3 + 3/4)
T3 = (48 / √3) * (7/4 - √3)
Simplifying:
T3 = (48 * 7 - 48√3) / (4√3)
T3 = (336 - 48√3) / (4√3)
T3 = (84 - 12√3) / √3
Multiplying the numerator and denominator by √3 to rationalize the denominator:
T3 = (84√3 -