Question 927495
1st row: 8 + 0
2nd row: 8 + 2
3rd row: 8 + 4
4th row: 8 + 6
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The formula for adding up the numbers:
1 + 2 + 3 + 4 + 5 + . . .  is:
{{{ n*( n+1 ) / 2 }}} where {{{ n }}}
is the last number
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The 1st term in your sequence is:
{{{ 50*8 = 400 }}}
The 2nd term in the sequence is:
2*( 1 + 2 + 3 + 4 +  . . . + 49 )
{{{ 2*49*50 / 2 = 2450 }}}
Note that the last term is {{{ 49 }}} because
the 1st term is {{{ 0 }}}
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{{{ 400 + 2450 = 2850 }}}
There are 2850 seats in the auditorium
Hope I got it -you might get a 2nd opinion, too