Question

If 3,5,7,9 are in AP then 20 terms

Answer 1

Answer:

an= a+(n-1)d

a20= 3+(20-1)2

a20=41

Explanation:

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Answer 2

Answer:

Solñ-

given,

a = 3

d = t1 - t2

= 5 - 3

= 2

an = 20

Now ,

an = a + (n+1) ×d

20 = 3 + (n +1)×2

20 = 3 + 2n + 2

20 = 5 + 2n

20 - 5 = 2n

15 = 2n

n = 15/2

n = 7.5

Then,

Sn = n/2 { a + ( n+ 1 ) × d}

= 7.5/2 { 3 + ( 7.5 + 1 ) × 2}

= 3.75 { 3 + 8.5 × 2 }

= 3.75 { 3 + 17 }

= 3.75 × 20

= 75