Question

For an arithmetic sequence, a26=80. If the common difference is 1, find:


a1=

the sum of the first 43 terms=

Answer 1

Step-by-step explanation:

Tn = a + (n-1) d

T26 = 80

d= 1

80= a + (26-1)*1

80 = a + 25

a = 80-25

a = 55

a1 = 55

Sn = 1/2{2a + (n-1)d}

S43 = 1/2 {2*55 + (43-1)*1}

= 1/2 (110 + 42)

= 55 + 21

= 76

S43 = 76

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