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## 'Look Before You Leap' printed from http://nrich.maths.org/

If $a+b+c=4$,

$ab+bc+ca=6$

and $abc=3$,

what are the values of:

${1\over a}+ {1\over b }+ {1\over c}$ (think of fractions),

${1\over ab}+ {1\over bc }+ {1\over ca}$

and $a^2 +b^2 + c^2$?

In the diagram the coloured squares have sides of length $a$, $b$
and $c$. Use the areas in the diagram to write down a formula for
the expansion of $(a + b + c)^2$ and explain your method.

Using your expansion of $(a + b + c)^2$ to help you, expand $(a + b
+ c)^3$. Can you explain each term of the expansion using a diagram
of a cube where each face has been cut up in a similar way to the
square above?