Question 924552
In the simplest form:
{{{ a*( b + c + d )  = a*b + a*c + a*d }}}
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If you prove it to yourself, it will
make more sense. Substitute numbers
for the letters:
{{{ 3*( 6 + 2 + 9 ) = 3*6 + 3*2 + 3*9 }}}
{{{ 3*( 6 + 2 + 9 ) = 18 + 6 + 27 }}}
{{{ 3*( 6 + 2 + 9 ) = 51 }}}
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check:
{{{ 3*( 6 + 2 + 9 ) = 3*17 }}}
{{{ 3*( 6 + 2 + 9 ) = 51 }}}
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This is a good proof -there are no numbers 
that fail this test- so you proved it