How do you find the 20th term and the sum of 20 first terms of the sequence 2, 5, 8, 11, 14, 17,...?

1 Answer
Jim G.
Jan 18, 2018

#a_(20)=59" and "S_(20)=610#

Explanation:

#"these are the terms of an "color(blue)"arithmetic sequence"#

#a,a+d,a+2d,a+3d,...... ,a+(n-1)d#

#"where a is the first term and r the common difference"#

#d=a_2-a_1=a_3-a_2=...... =a_n-a_(n-1)#

#• " the n th term is "a_n=a+(n-1)d#

#• " the sum of n terms is "S_n=n/2[2a+(n-1)d]#

#d=5-2=8-5=11-8=3#

#rArra_(20)=2+(19xx3)=2+57=59#

#rArrS_(20)=10(4+57)=610#

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