find the sum of the first 40 terms of the arithmetic sequence 7,15,23,31,...
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find the sum of the first 40 terms of the arithmetic sequence 7,15,23,31,...
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Answer:
6520
Step-by-step explanation:
15-7=8
23-15=8
31-23=8
common difference=8
a sub n = a sub 1 + (n-1)d
a sub 40 = 7 + (40-1)8
a sub 40 = 7 + (39)8
a sub 40 = 7 + (312)
a sub 40 = 319
S sub n = n/2(a sub 1 + a sub n)
S sub 40 =40/2(7+319)
S sub 40 =20(326)
S sub 40 = 6520