Question 983880
The numbers are:
3, 6, 9, 12, 15 . . . 99
I can express the sum as:
 1*3 +2*3 + 3*3 + 4*3 + 5*3 + . . . + 33*3
Also as:
3*( 1 + 2 + 3 + 4 + 5 + . . . + 33 )
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The formula for the sum of numbers from
1 to n is: {{{ S = (1/2)*( n*( n+1 )) }}}
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for instance:
{{{ 1 + 2 + 3 + 4 +5 = 15 }}}
{{{ S = (1/2)*( 5*6 ) }}}
{{{ S = 15 }}}
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3*( 1 + 2 + 3 + 4 + 5 + . . . + 33 ) = {{{ 3*(1/2)*( 33*34 ) }}}
{{{ S = ( 3/2)*1122 }}}
{{{ S = 3*561 }}}
{{{ S = 1683 }}}
Hope I got it!