document.write( "Question 864515: Dominoes are stacked on top of each other such that the first row contains 80, the 2nd row 78, etc so that each row contains 2 fewer dominoes than the previous row. If the number of dominoes in the pile totals 710, how many rows are there in the stack? \n" ); document.write( "

Algebra.Com's Answer #521099 by richwmiller(17219) $\"\"$ $\"About$

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d=-2 not +2\r
\n" ); document.write( "\n" ); document.write( "710 = ½(160+ (n-1)(-2))n
\n" ); document.write( "710 = ½(160-2n+2))n\r
\n" ); document.write( "\n" ); document.write( "n^2-81n+710 = 0
\n" ); document.write( "n = 10
\n" ); document.write( "S = ½(2a + (n-1)d)n
\n" ); document.write( "S = ½(2*80 + (9)-2)10
\n" ); document.write( "S = ½(160 -18)10
\n" ); document.write( "S = 5*(142)
\n" ); document.write( "s=710\r
\n" ); document.write( "
\n" ); document.write( "\n" ); document.write( "80,78,76,74,72,70,68,66,64,62
\n" ); document.write( "80+78+76+74+72+70+68+66+64+62 = 710
\n" ); document.write( "an = a + (n - 1)d
\n" ); document.write( "a10 = 80 + (9)(-2)=62\r
\n" ); document.write( "
\n" ); document.write( "\n" ); document.write( "n = 71
\n" ); document.write( "S = ½(160-2*(70))71
\n" ); document.write( "S=1/2(20)*71
\n" ); document.write( "s=10*71
\n" ); document.write( "A71 = 80 + (70)-2
\n" ); document.write( "A71=80-140
\n" ); document.write( "a71=-60
\n" ); document.write( "n=71 is an extraneous root \n" ); document.write( "

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