4 Views

Question : If $a=\sqrt{6}+\sqrt{5},b=\sqrt{6}-\sqrt{5}$, then $2a^{2}-5\; ab+2b^{2} =?$

Option 1: 38

Option 2: 39

Option 3: 40

Option 4: 41


Team Careers360 19th Jan, 2024
Answer (1)
Team Careers360 20th Jan, 2024

Correct Answer: 39


Solution :
Given: $a=\sqrt{6}+\sqrt{5}$ and $b=\sqrt{6}-\sqrt{5}$
Then,
$a^{2} = (\sqrt{6}+\sqrt{5})^{2} = 6 + 2\sqrt{30} + 5 = 11 + 2\sqrt{30}$
$b^{2} = (\sqrt{6}-\sqrt{5})^{2} = 6 - 2\sqrt{30} + 5 = 11 - 2\sqrt{30}$
Now, the product of $a$ and $b$:
$ab = (\sqrt{6}+\sqrt{5})(\sqrt{6}-\sqrt{5}) = 6 - 5 = 1$
Putting the values into the given expression:
$2a^{2}-5ab+2b^{2} = 2(11 + 2\sqrt{30}) - 5(1) + 2(11 - 2\sqrt{30})$
Simplifying this, we get:
$= 22 + 4\sqrt{30}- 5 + 22 - 4\sqrt{30}$
$=44-5 = 39$
Hence, the correct answer is 39.

Know More About

Related Questions

TOEFL ® Registrations 2024
Apply
Accepted by more than 11,000 universities in over 150 countries worldwide
Manipal Online M.Com Admissions
Apply
Apply for Online M.Com from Manipal University
View All Application Forms

Download the Careers360 App on your Android phone

Regular exam updates, QnA, Predictors, College Applications & E-books now on your Mobile

150M+ Students
30,000+ Colleges
500+ Exams
1500+ E-books