How to minimize $ab + bc + ca$ given $a^2 + b^2 + c^2 = 1$? The question is to prove that $ab + bc + ca$ lies in between $-1$ and $1$, given that $a^2 + b^2 + c^2 = 1$.
I could prove the maxima by the following approach. I changed the coordinates to spherical coordinates:
$a = \cos A \\
b = \sin A \cos B \\
c = \sin A \sin B$
Using that $\cos X + \sin X$ always lies between $- \sqrt 2$ and $\sqrt 2$ I proved that $a + b + c$ lies between $- \sqrt 3$ and $\sqrt 3$.
Using $(a + b + c)^2 = a^2 + b^2 + c^2 + 2(ab + bc + ca)$ I could prove that $ab + bc + ca$ is maximum at $1$ but I can't prove the minima.

The powerful Rayleigh quotient method can solve this problem, and many similar ones.
Indeed, this issue is equivalent to the maxi/minimization of the (Rayleigh) quotient $q(x)=\dfrac{ab+bc+ca}{a^2+b^2+c^2}=\dfrac{X^TAX}{X^TX}$ with $A=\dfrac{1}{2}\begin{pmatrix} 0 & 1 & 1 \\ 1 & 0 & 1 \\ 1 & 1 & 0 \end{pmatrix}$.
The general result is that such a  quotient takes its values in the closed interval $[\lambda_{min},\lambda_{max}]$ where $\lambda_{min}$ and $\lambda_{max}$ are resp. the minimal and maximal eigenvalues of $A$ and values $(a,b,c)$ providing the min (resp. max) are given by the associated (normed) eigenvectors.
Easy computations show that the eigenvalues of $A$ are $-1/2$ (multiplicity 2) and $1$, with associated normed eigenvectors: 


*

*for $\lambda=-1/2$: $(a,b,c)=\dfrac{1}{\sqrt{2}}(-1,0,1)$ and $(a,b,c)=\dfrac{1}{\sqrt{2}}(-1,1,0)$, 

*for $\lambda=1$: $(a,b,c)=\dfrac{1}{\sqrt{3}}(1,1,1)$.
Thus the interval of values taken by $ab+bc+ca$ is in between $\lambda_{min}=-0.5$ and $\lambda_{max}=1$, these extreme values being reached for vectors $(a,b,c)$ as given above.
For more on the Rayleigh quotient method, see https://en.wikipedia.org/wiki/Rayleigh_quotient ; for statistical applications, have a look at the slides "Rayleigh quotient" in http://www.svcl.ucsd.edu/courses/ece271B-F09/
Besides this algebraic approach, there is a geometric interpretation: let $S_k$ be the surface with equation $xy+yz+zx=k$. The maximum and minimum values for $k$ correspond to the limit cases for which $S_k \cap B \neq \varnothing$ where $B$ denotes the unit sphere. 
