Quantitative Aptitude – Modern Maths – Sequence and Series – The value of the sum 7 x 11
Slot -2 – Quantitative Aptitude – Modern Maths – Sequence and Series – The value of the sum 7 x 11
The value of the sum 7 x 11 + 11 x 15 + 15 x 19 + …+ 95 x 99 is?
a) 80707
b) 80730
c) 80773
d) 80751
Answer: a) 80707
Solution: 7 x 11 + 11 x 15 + 15 x 19 + …+ 95 x 99
nth term of series = {4(n+1)-1}×{ 4 (n+1)+3}=16n^2+16+32n+8n+8-
3=16n^2+40n+21
Thus 7 x 11 + 11 x 15 + 15 x 19 + …+ 95 x 99 = ∑ Tn=∑ (16n^2+40n+15)
Required sum = ∑ Tn=∑ (16n^2+ 40n+15)=16 ×(n(n+1(2n+1))/6 + 40×n(n+1)/2 + 21n
Putting n= 23
Sum = 16×23×24×47/6+40×23×24/2+21×23
=69184+11040+483=80707
Correct option a) 80707
Other posts related to Quantitative Aptitude – Modern Maths
a) 1000+ Videos covering entire CAT syllabus b) 2 Live Classes (online) every week for doubt clarification c) Study Material & PDFs for practice and understanding d) 10 Mock Tests in the latest pattern e) Previous Year Questions solved on video