# Proof of Cauchy's Inequality ... Sohrab Proposition 2.1.23

1. Aug 7, 2017

### Math Amateur

1. The problem statement, all variables and given/known data

I am reading Houshang H. Sohrab's book: "Basic Real Analysis" (Second Edition).

I am focused on Chapter 2: Sequences and Series of Real Numbers ... ...

I need help with Proposition 2.1.23/Exercise 2.1.24 (Exercise 2.1.24 asks readers to prove Proposition 2.1.23)

Proposition 2.1.23/Exercise 2.1.24 reads as follows:

In the above text by Sohrab, we read the following:

" ... ... Observe that for any $t \in \mathbb{R}, F(t) = \sum_{ i = 1 }^n ( x_i - t y_i )^2 \ge 0$ and look at the discriminant $Z^2 - XY$ of $F(t)$. ... ... "

Can someone please explain how one proceeds after determining the discriminant and what principles/properties of the discriminant are used ...

2. Relevant equations

The relevant information is given in the problem statement ... basic properties of the real numbers are assumed ...

3. The attempt at a solution

my working so far on the exercise is as follows:

$F(t) = \sum_{ i = 1 }^n ( x_i - t y_i )^2 \ge 0$

Now ... ... $\sum_{ i = 1 }^n ( x_i - t y_i )^2 \ge 0$

$\Longleftrightarrow \sum_{ i = 1 }^n ( x_i^2 - 2t x_i y_i + t^2 y_i^2) \ge 0$

$\Longleftrightarrow \sum_{ i = 1 }^n x_i^2 - 2 t \sum_{ i = 1 }^n x_i y_i + t^2 \sum_{ i = 1 }^n y_i^2 \ge 0$ ... ... ... (1)

... now let $X = \sum_{ i = 1 }^n x_i^2$, $\$ $Y = \sum_{ i = 1 }^n y_i^2 \$ and $\ Z = \sum_{ i = 1 }^n x_i y_i$

Then (1) becomes $X - 2t Z + Y t^2 \ge 0$

and the discriminant, $D = 4Z^2 - 4XY$

BUT ... how do we proceed from here .. ... and exactly what properties of the discriminant do we use ...

Help will be appreciated,

Peter

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• ###### Sohrab - Cauchy's Inequality - Proposition 2.1.23 and Exercise 2.1.24 ....png
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Last edited: Aug 7, 2017
2. Aug 7, 2017

### pasmith

If $ax^2 + bx + c \geq 0$ for all $x \in \mathbb{R}$, then its roots are either both zero or a complex conjugate pair.

3. Aug 8, 2017

### Math Amateur

Thanks pasmith ... BUT ... I am not sure how to use that fact ...

Peter

EDIT

On further reflection, then the discriminant is less than or equal to zero ... so the result follows ...

I think that that is correct ...

Thanks ...

Last edited: Aug 8, 2017