Question

given a=7,a13 = 35, find d and S13
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Answer 1

Step-by-step explanation:

a13=35

we know that an=a+(n-1)d

therefore,

a+(13-1)d=35

a+12d=35

And a=7

7+12d=35

12d=35-7

12d=28

d=28/12

d=7/3

S13=13/2[2a+(13-1)d]

putting the value of d and a in the equation

S13= 13/2[2(7) + 12(7/3)]

S13=13/2[14+28]

S13=13/2 × 42

S13=13×21

S13=273