JEE MAIN - Mathematics (2025 - 3rd April Morning Shift - No. 2)
Explanation
We start with the given sequence $a_1, a_2, a_3, \ldots$ of an increasing geometric progression (G.P.). Two key conditions are provided:
$a_3 \cdot a_5 = 729$
$a_2 + a_4 = \frac{111}{4}$
Calculating using given conditions:
Using the sequence terms:
$ a_3 = a \cdot r^2, \quad a_5 = a \cdot r^4, $
Then:
$ a_3 \cdot a_5 = (a \cdot r^2)(a \cdot r^4) = a^2 \cdot r^6 = 729 = 27^2 $
It follows:
$ a \cdot r^3 = 27 \quad \Rightarrow \quad a_4 = a \cdot r^3 = 27 \quad \text{(i)} $
Using the second condition:
$ a_2 + a_4 = \frac{111}{4} $
Substituting $a_4 = 27$:
$ a_2 = \frac{111}{4} - 27 $
$ a_2 = a \cdot r = \frac{3}{4} \quad \text{(ii)} $
Solving for $r$ and $a$:
From equation (i) and (ii), we derive $r^2$:
$ r^2 = \frac{4 \cdot 27}{3} = 4 \times 9 = 36 $
Since the G.P. is increasing, $r = 6$.
Solve for $a$:
Substituting $r = 6$ into $a \cdot r = \frac{3}{4}$:
$ a \cdot 6 = \frac{3}{4} \quad \Rightarrow \quad a = \frac{3}{24} = \frac{1}{8} $
Calculating the requested sum:
The task is to find:
$ 24(a_1 + a_2 + a_3) = 24(a + ar + ar^2) $
Substitute the values:
$ = 24 \times \frac{1}{8}(1 + 6 + 6^2) = 3 \times (1 + 6 + 36) = 3 \times 43 = 129 $
Therefore, the value is 129.
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