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

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.

Solution:

1. first row = 40
2. second row = 38
3. third row = 36

so,

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