Question

if a7=6 in an ap of 13 terms then find the value of s13

Answer 1

I hope it will help you

Answer 2

Answer:

Here a7=6

a7 can be written as a+6d

So,

a+6d=6

When 6d goes to right hand side it becomes a=6–6d

Take a=6–6d as eq 1

a13=a+12d

6–6d+12d

6=–6d

d=–1

Here d=–1 . Substitute it in eq=1

a=6–6d

a=6–6(–1)

a=6+6

a=12

Substitute a and d on the sum formula

Sn=n/2(2a+(n–1)d)

S13=13/2(2(12)+(13–1)–1)

S13=13/2(24–12)

S13=13/2(12)

S13=13×6

S13=78

Hence the value of S13 is 78

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