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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