Question

an ap consists of 21 terms the sum of the three terms in the middle is 63 and sum of last 3 terms is 90 find the AP

Answer 1

The three middle terms are 10th, 11th and 12 th

According to question
T10 + T11 +T12 = 63
=> a+9d +a +10d +a +11d = 63
=> 3a +30d = 63
=> a+10d = 21 -------(1)

Now,
T19 + T20 +T21 = 90
=> a+18d +a +19d +a + 20d = 90
=> 3a +57d = 90
=> a +19d = 30 --------(2)


On subtracting equation 1 from 2, we get
9d = 9

d = 1
a = 11

Required AP is 11, 12, 13,.........upto 21 terms

Answer 2

Answer:

Step-by-step explanation:

The three middle terms are 10th, 11th and 12 th

According to question

T10 + T11 +T12 = 63

=> a+9d +a +10d +a +11d = 63

=> 3a +30d = 63

=> a+10d = 21 -------(1)

Now,

T19 + T20 +T21 = 90

=> a+18d +a +19d +a + 20d = 90

=> 3a +57d = 90

=> a +19d = 30 --------(2)

On subtracting equation 1 from 2, we get

9d = 9

d = 1

a = 11

Required AP is 11, 12, 13,....21 terms