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?
Found 2 solutions by ewatrrr, richwmiller:
Answer by ewatrrr(24785)   (Show Source):
Answer by richwmiller(17219)  (Show Source):
You can put this solution on YOUR website! d=-2 not +2
710 = ½(160+ (n-1)(-2))n
710 = ½(160-2n+2))n
n^2-81n+710 = 0
n = 10
S = ½(2a + (n-1)d)n
S = ½(2*80 + (9)-2)10
S = ½(160 -18)10
S = 5*(142)
s=710
80,78,76,74,72,70,68,66,64,62
80+78+76+74+72+70+68+66+64+62 = 710
an = a + (n - 1)d
a10 = 80 + (9)(-2)=62
n = 71
S = ½(160-2*(70))71
S=1/2(20)*71
s=10*71
A71 = 80 + (70)-2
A71=80-140
a71=-60
n=71 is an extraneous root
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