Difference AP

Calculate the difference of arithmetic progression if the sum of its first 19 members Sn = 8075 and the first member is a₁ = 20

Correct answer:

d =  45

Step-by-step explanation:

$$\begin{gathered} S=n{\displaystyle \frac{a_1+a_n}{2}} \\ 8075=19{\displaystyle \frac{20+20+(19-1)d}{2}} \\ 850=2\cdot 20+(19-1)d \\ 810=18d \\ d=45 \\ S=20+65+110+155+200+245+290+335+380+425+470+515+560+605+650+695+740+785+830=8075 \end{gathered}$$

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