1. ## Cauchy-Schwarz Inequality

If $\displaystyle a_{1}, \ldots, a_{n}$ and $\displaystyle b_{1} \ldots, b_{n}$ are arbitrary real numbers, then $\displaystyle \left(\sum_{k=1}^{n} a_{k}b_{k} \right)^{2} \leq \left(\sum_{k=1}^{n} a_{k}^{2} \right) \left(\sum_{k=1}^{n} b_{k}^{2} \right)$. If some $\displaystyle a_{i} \neq 0$ then equality holds if and only if there is a real $\displaystyle x$ such that $\displaystyle a_{k}x + b_{k} = 0$ for each $\displaystyle k = 1,2, \ldots , n$.

So to basically prove this you do the following: $\displaystyle \sum_{k=1}^{n} (a_{k}x + b_{k})^{2} \geq 0$ for all $\displaystyle x$. It is equaled to $\displaystyle 0$ if $\displaystyle x_i = 0$.

And this is equaled to $\displaystyle \sum_{k=1}^{n} a_{k}x^{2} + 2a_{k}x b_{k} + b_{k}^{2} \geq 0$

Then we invoke the discriminant of the quadratic equation $\displaystyle Ax^{2} + 2Bx + C \geq 0$ by making the required substitutions?

2. Originally Posted by heathrowjohnny
If $\displaystyle a_{1}, \ldots, a_{n}$ and $\displaystyle b_{1} \ldots, b_{n}$ are arbitrary real numbers, then $\displaystyle \left(\sum_{k=1}^{n} a_{k}b_{k} \right)^{2} \leq \left(\sum_{k=1}^{n} a_{k}^{2} \right) \left(\sum_{k=1}^{n} b_{k}^{2} \right)$. If some $\displaystyle a_{i} \neq 0$ then equality holds if and only if there is a real $\displaystyle x$ such that $\displaystyle a_{k}x + b_{k} = 0$ for each $\displaystyle k = 1,2, \ldots , n$.

So to basically prove this you do the following: $\displaystyle \sum_{k=1}^{n} (a_{k}x + b_{k})^{2} \geq 0$ for all $\displaystyle x$. It is equaled to $\displaystyle 0$ if $\displaystyle x_i = 0$.

And this is equaled to $\displaystyle \sum_{k=1}^{n} a_{k}x^{2} + 2a_{k}x b_{k} + b_{k}^{2} \geq 0$

Then we invoke the discriminant of the quadratic equation $\displaystyle Ax^{2} + 2Bx + C \geq 0$ by making the required substitutions?

have you ever tried searching from google or yahoo? in any field, the cauch-schwarz inequality have the same proof..

correction: $\displaystyle \sum_{k=1}^{n} {a_{k}}^2x^{2} + 2a_{k}x b_{k} + b_{k}^{2} \geq 0$

anyways to continue your proof, yes, you must consider the discriminant, and that the discriminant must be $\displaystyle \leq 0$ (why?) (since $\displaystyle Ax^{2} + 2Bx + C \geq 0$, that is, if it is > 0, the equation has no real root and so the discriminant is < 0. ...)
can you continue now??