How do you find the sum of the arithmetic sequence given an = 5, d = -2, n = 12?

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Answer 1 · 2015-06-24T15:12:46

# =color(blue)(192#

We have: #an = 5# , #d=-2# and #n=12#

By formula:
# color(blue)(an = a1+ (n-1)d#
# 5 = a1+(12-1) . (-2)#
# 5 = a1 + 11. (-2)#
# 5 = a1-22#
# a 1= 27#

Now,
The formula for sum of an Arithmetic Sequence when the first term #a1#, number of terms #n# and last term #an# are given is:

# color(blue)(Sum = (n/2)(a1+an)#

# = 12/2 (27 + 5)#

# = 6 xx 32#

# =color(blue)(192#