Posted by **Riana** on Tuesday, January 1, 2013 at 7:56pm.

bricks are stacked in 20 rows such that each row has a fixed number of bricks more than the row above it. the top row has 5 bricks and the bottom row 62 bricks. how many bricks are in the stack?

- math -
**Reiny**, Tuesday, January 1, 2013 at 8:03pm
using your arithmetic sequence definitions

t1 = a = 5

t20 = a+19d = 62

5+19d = 62

19d = 57

d = 3

sum(20) = (20/2)(first + last)

= 10(5+62) = 670

- math -
**Riana**, Tuesday, January 1, 2013 at 8:15pm
how did you gt 19d ?

- math -
**Reiny**, Tuesday, January 1, 2013 at 8:23pm
The formula for

term(n) = a + (n-1)d

so term(20) = a+19d

You MUST know these to be able to work with sequences

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