JEE MAIN - Mathematics (2022 - 26th June Evening Shift - No. 20)
If a1 (> 0), a2, a3, a4, a5 are in a G.P., a2 + a4 = 2a3 + 1 and 3a2 + a3 = 2a4, then a2 + a4 + 2a5 is equal to ___________.
Answer
40
Explanation
Let G.P. be a1 = a, a2 = ar, a3 = ar2, .........
$$\because$$ 3a2 + a3 = 2a4
$$\Rightarrow$$ 3ar + ar2 = 2ar3
$$\Rightarrow$$ 2ar2 $$-$$ r $$-$$ 3 = 0
$$\therefore$$ r = $$-$$1 or $${3 \over 2}$$
$$\because$$ a1 = a > 0 then r $$\ne$$ $$-$$1
Now, a2 + a4 = 2a3 + 1
ar + ar3 = 2ar2 + 1
$$a\left( {{3 \over 2} + {{27} \over 8} - {9 \over 2}} \right) = 1$$
$$\therefore$$ a = $${8 \over 3}$$
$$\therefore$$ a2 + a4 + 2a5 = a(r + r3 + 2r4)
$$ = {8 \over 3}\left( {{3 \over 2} + {{27} \over 8} + {{81} \over 8}} \right) = 40$$
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