Question

If d equal to 2/3 ,l equals to 10 and n equal to 20 then find a and Sn

Answer 1

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Sn = n/2[2×10 + (n-1) 2/3]

• = n/2[20 + 2/3n - 2/3]
• = n/2[58/3 + 2/3n]
• = 29n/3 +n²/3