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Arithmetic Progressions

Question
CBSEENMA10007981

In a flower bed, there are 23 rose plants in the first row, 21 in the second, 19 in the third and so on. There are 5 rose plant in the last row. How many rows are there in the flower bed?

Solution

The number of rose plants in the rows are 23, 21, 19, .......... 5
Here, a = 23, d = –2, an = 5
We know that,an = a + (n – 1)d
⇒ 5 = 23 + (n – 1) (–2)
⇒ 5 = 23 – 2n + 2
⇒ 5 = 25 – 2n
⇒ 2n = –20
⇒ n = 10
Hence, there are 10 rows in the flower bed.