# Difference between revisions of "Cauchy-Schwarz Inequality"

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<math> \displaystyle (a_1x+b_1)^2+(a_2x+b_2)^2+...+(a_nx+b_n)^2=0</math>. | <math> \displaystyle (a_1x+b_1)^2+(a_2x+b_2)^2+...+(a_nx+b_n)^2=0</math>. | ||

− | Expanding, we find the equation to be of the form <math>Ax^2+Bx+C</math> | + | Expanding, we find the equation to be of the form |

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+ | <math> \displaystyle Ax^2 + Bx + C,</math> | ||

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+ | where <math>A=\sum_{i=1}^n a_i^2</math>, <math>B=2\sum_{j=1}^n a_jb_j</math>, and <math>C=\sum_{k=1}^n b_k^2.</math>. By the [[Trivial inequality | Trivial Inequality]], we know that the left-hand-side of the original equation is always at least 0, so either both roots are [[Complex Numbers]], or there is a double root at <math>x=0</math>. Either way, the [[discriminant]] of the equation is nonpositive. Taking the [[discriminant]], <math>B^2-4AC \leq 0</math> and substituting the above values of A, B, and C leaves us with the '''Cauchy-Schwarz Inequality''', <math>(a_1b_1+a_2b_2+...+a_nb_n)^2 \leq (a_1^2+a_2^2+...+a_n^2)(b_1^2+b_2^2+...+b_n^2)</math>, | ||

or, in the more compact [[sigma notation]], | or, in the more compact [[sigma notation]], | ||

<math>\left(\sum a_ib_i\right)</math> <math>\leq \left(\sum a_i^2\right)\left(\sum b_i^2\right)</math> | <math>\left(\sum a_ib_i\right)</math> <math>\leq \left(\sum a_i^2\right)\left(\sum b_i^2\right)</math> |

## Revision as of 01:30, 18 June 2006

The **Cauchy-Schwarz Inequality** (which is known by other names, including Cauchy's Inequality) states that, for two sets of real numbers and , the following inequality is always true:

Equality holds if and only if .

There are many ways to prove this; one of the more well-known is to consider the equation

.

Expanding, we find the equation to be of the form

where , , and . By the Trivial Inequality, we know that the left-hand-side of the original equation is always at least 0, so either both roots are Complex Numbers, or there is a double root at . Either way, the discriminant of the equation is nonpositive. Taking the discriminant, and substituting the above values of A, B, and C leaves us with the **Cauchy-Schwarz Inequality**, ,
or, in the more compact sigma notation,

Note that this also gives us the equality case; equality holds if and only if the discriminant is equal to 0, which is true if and only if the equation has 0 as a double root, which is true if and only if .

This inequality is used very frequently to solve Olympiad-level Inequality problems, such as those on the USAMO and IMO.