If 1+ √3-√2 2 √3+ 5-2 √6 18+ 9 √3-11 √2 36 √3+ 49-20 √6 180+… upto ∞=2+ (√ b a+1 ) log _e ( a b ), where a and b are integers with gcd( a, b)=1, then…
JEE Main 2024 — Mathematics Algebra
2024integermedium
If 1+233−2+185−26+36393−112+18049−206+… upto ∞=2+(ab+1)loge(ba), where a and b are integers with gcd(a,b)=1, then 11a+18b is equal to ______
Official previous-year question
Held on 5 Apr 2024 · Verified 6 Jul 2026.
Did you get this right?
Sign in to track your attempts and accuracy.
Solution
S=1+23x+18x2+363x3+180x4+…∞ Put 3x=t, where x=3−2S=1+2t+6t2+12t3+20t4+…S=1+t(1−21)+t2(21−31)+t3(31−41)+t4(41−51)S=(1+t+2t2+3t3+4t3+…)−(2t+3t2+4t3+5t4+…)S=(t+2t2+…)−t1(t+2t2+3t3+…)+2S=2+(1−t1)(−log(1−t))=(t1−1)log(1−t)+2S=2+(3−23−1)log(1−33−2)S=2+(3−22)loge32S=2+2(6+2)loge32=2+(23+1)loge32a=2,b=311a+18b=11×2+18×3=76
Your note
Sign in to keep a private note on this question. Nothing you write is ever public.