JEE Advance - Mathematics (2016 - Paper 2 Offline - No. 15)
Explanation
If logb1, logb2, ......, logb101 are in A.P. with common difference loge2, then b1, b2, ......, b101 are in G.P., with common ratio 2.
$$\therefore$$ b1 = 20b1
b2 = 21b1
b3 = 22b1
$$\matrix{ \vdots & \vdots & \vdots \cr } $$
b101 = 2100b1 ..... (i)
Also, a1, a2, ......, a101 are in A.P.
Given, a1 = b1 and a51 = b51
$$\Rightarrow$$ a1 = b1 and a51 = b51
$$\Rightarrow$$ a1 + 50D = 250 b1
$$\Rightarrow$$ a1 + 50D = 250 a1 [$$\because$$ a1 = b1] ...... (ii)
Now, t = b1 + b2 + ..... + b51
$$ \Rightarrow t = {b_1}{{({2^{51}} - 1)} \over {2 - 1}}$$ ..... (iii)
and s = a1 + a2 + .... + a51
$$ = {{51} \over 2}(2{a_1} + 50D)$$ ...... (iv)
$$\therefore$$ t = a1(251 $$-$$ 1) [$$\because$$ a1 = b1]
or t = 251 a1 $$-$$ a1 < 251 a1 ...... (v)
and $$s = {{51} \over 2}[{a_1} + ({a_1} + 50D)]$$ [from Eq. (ii)]
$$ = {{51} \over 2}[{a_1} + {2^{50}}{a_1}] = {{51} \over 2}{a_1} + {{51} \over 2}{2^{50}}{a_1}$$
$$\therefore$$ s > 251 a1 ...... (vi)
From Eqs. (v) and (vi),
we get s > t
Also, a101 = a1 + 100 D
and b101 = 2100 b1
$$\therefore$$ $${a_{101}} = {a_1} + 100\left( {{{{2^{50}}{a_1} - {a_1}} \over {50}}} \right)$$
and b101 = 2100 a1
$$\Rightarrow$$ a101 = a1 + 251 a1 $$-$$ 2a1 = 251 a1 $$-$$ a1
$$\Rightarrow$$ a101 < 251 a1
and b101 > 251 a1 $$\Rightarrow$$ b101 > a101
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