If x2 + y2 = 29 and x - y = 3, then what is the value of 4xy?
Show answer & explanation
Correct answer: A
Explanation
The correct answer is option 1 i.e. 40.
x2 + y2 = 29
x − y = 3
Now square both sides of (x − y = 3):
(x − y)2 = 9
⇒ x2 − 2xy + y2 = 9
Substitute x2 + y2 = 29:
29 − 2xy = 9
⇒ 2xy = 20
⇒ 4xy = 40