The 15th term of the series 1 + 4 + 7 + 10 is? Where the sum of 18 terms is:
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an =a+(n-1)d
a=1 d= 4-1=3
a15 =a+(n-1)d
a15=1+(15-1)3
a15=1+(14)3
a15= 1+42
a15=43
sn=n/2(2a+(n-1))d)
s18=18/2(2×1+(18-1)3
s18=9(2+(17)3)
s18=9(2+58)
s18=9×60
s18=540
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