How do you find the sum of the arithmetic sequence having the data given #a_1=7#, d = - 3, n = 20?

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Answer 1 · 2016-03-05T16:35:33

The sum # color(blue)(= -430#

The formula for the sum of an arithmetic sequence when #a_1 , d and n# are given is:

#color(blue)(Sum = (n/2)[2a + (n-1)d]#

Data provided is:
#a_1=7, d=-3, n=20#

Here :# a_1 = 7, d=-3 and n=20#

# = (20/2)[2 . 7 + (20 - 1) . -3]#

# = 10 . [14+ 19 .( -3)]#

# = 10 . [14 -57]#

# color(blue)(= -430#