PROBLEM/SITUATION: 1.) Prove that the product xy of two real numbers x and y is nonnegative if and only if the absolute value |x +y| of their sum is the sum |x| + |y| of their absolute values.

Algebra & Trigonometry with Analytic Geometry
13th Edition
ISBN:9781133382119
Author:Swokowski
Publisher:Swokowski
Chapter2: Equations And Inequalities
Section2.6: Inequalities
Problem 80E
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PROBLEM/SITUATION:

1.) Prove that the product xy of two real numbers x and y is nonnegative if and only if the absolute value |x +y| of their sum is the sum |x| + |y| of their absolute values. 

 

For no.1:

Recall that |x| = x iff x >= 0

Show ab < 0 leads to a contradiction.

Then assume ab>=0.

Prove by cases:

(i) Let ab=0.

(ii) Let ab > 0

 

NOTE: Type only your answers. Please do not handwritten your answers. Make sure your formulas, solutions and answers' format are all correct.  

 

Step 1
We have to prove that product xy of two real
numbers x and y is nonnegative if and only if the
absolute value |x + y of their sum is the sum |x| + |y|
of their absolute values
Step 2
If part :-
Let product xy of two real numbers x and y is
nonnegative then we have to prove that absolute
value x + y of their sum is the sum |x| + |y| of their
absolute values
By triangle inequality
|x + y ≤ |x|+|yl|
Equality hods if both x and y is positive or both x
and y is negative or x = 0 or y = 0
Since,
xy is nonnegative
So,
both x and y is positive or both x and y is negative
or x = 0 ory=0
Hence,
|x + y = |x|+|y|
Step 3
Only if part :-
Let |x + y = |x|+|y| then we have to prove that xy is
nonnegative
(x + y)² = (x + y)²
x² + 2xy + y² = x² +2|x||y| + y² = x² + 2|xy| + y²
xy = |xy|
xy ≥ 0
So,
xy is nonnegative
Hence,
Product xy of two real numbers x and y is
nonnegative if and only if the absolute value
|x + y of their sum is the sum |x| + |y| of their
absolute values
Transcribed Image Text:Step 1 We have to prove that product xy of two real numbers x and y is nonnegative if and only if the absolute value |x + y of their sum is the sum |x| + |y| of their absolute values Step 2 If part :- Let product xy of two real numbers x and y is nonnegative then we have to prove that absolute value x + y of their sum is the sum |x| + |y| of their absolute values By triangle inequality |x + y ≤ |x|+|yl| Equality hods if both x and y is positive or both x and y is negative or x = 0 or y = 0 Since, xy is nonnegative So, both x and y is positive or both x and y is negative or x = 0 ory=0 Hence, |x + y = |x|+|y| Step 3 Only if part :- Let |x + y = |x|+|y| then we have to prove that xy is nonnegative (x + y)² = (x + y)² x² + 2xy + y² = x² +2|x||y| + y² = x² + 2|xy| + y² xy = |xy| xy ≥ 0 So, xy is nonnegative Hence, Product xy of two real numbers x and y is nonnegative if and only if the absolute value |x + y of their sum is the sum |x| + |y| of their absolute values
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