Question

390 plants are to be planted in a garden in a number of rows. There are 40 plants in the 1" row, 38 plants in the
second row, 36 plants in 3 row and so on. In how many rows the 390 plants are planted ? Find the no of plants in
the last row also

Answer 1

Step-by-step explanation:

first row = 40

second row = 38

third row = 36

....... so on

they are planted in ap series.

a = 40

d = 2

Sn = 390

n = ?

Sn = n/2[2a+(n-1)d]

390 = n/2[2*40+(n-1)2]

780 = n[80+2n-2]

780 = 78n + 2n^2

2n^2 + 78n - 780 = 0

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