Skip to main contentSkip to solution

The 21st and 33rd terms of an arithmetic progression are 91 and 145 respectively. What is the 29th term?

Solution

✅ Correct Option: 1

An arithmetic progression has the formula:

an=a+(n−1)da_n = a + (n - 1)d

Where aa is the first term, nn is the position, and dd is the common difference.

Given that the 21st term is 91 and the 33rd term is 145.


Using the formula for both given terms:

a21=a+20d=91a_{21} = a + 20d = 91 ... ①

a33=a+32d=145a_{33} = a + 32d = 145 ... ②


Subtracting equation ① from equation ②:

(a+32d)−(a+20d)=145−91(a + 32d) - (a + 20d) = 145 - 91

12d=5412d = 54

d=5412d = \frac{54}{12}

d=4.5d = 4.5


Substituting d=4.5d = 4.5 into equation ①:

a+20(4.5)=91a + 20(4.5) = 91

a+90=91a + 90 = 91

a=1a = 1


Finding the 29th term:

a29=a+(29−1)da_{29} = a + (29 - 1)d

a29=1+28(4.5)a_{29} = 1 + 28(4.5)

a29=1+126a_{29} = 1 + 126

a29=127a_{29} = 127

Therefore, the 29th term is 127.

The answer is option 1.

Keyboard Shortcuts

  • Left arrow: Previous question
  • Right arrow: Next question
  • S key: Jump to solution
  • Q key: Jump to question