How do you find the 12th partial sum of the arithmetic sequence #4.2, 3.7, 3.2, 2.7,...#?

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Answer 1 · 2017-02-24T17:06:31

#20.4#

#a_0 = 4.2#

#a_n = a_0 + (n - 1) (-0.5)#

#a_11 = 4.2 - 0.5 * 10 = -0.8#

#a_0 + a_11 = 3.4#

#a_1 + a_10 = 3.4#

#a_2 + a_9 = 3.4#

#a_3 + a_8 = 3.4#

#a_4 + a_7 = 3.4#

#a_5 + a_6 = 3.4#

#s_11 = a_0 + ... + a_11 = 3.4 * 6#