NCERT Solutions
Class 11 Maths
Sequences and Series

Ex.Misc.Q.1
The mean and variance of eight observations are 9 and 9.25, respectively.
six of the observations are 6, 7, 10, 12, 12 and 13, find the remaining two observations.
Let the remaining two observations be x and y.
Therefore, the observations are 6, 7, 10, 12, 12, 13, x, y.
Mean, x = (6 + 7 + 10 + 12 + 12 + 13 + x + y) ÷ 8 = 9
=>60 + x + y = 9 × 8
=> 60 + x + y = 72
=> x + y = 72 – 69
=> x + y = 12 ……………… (1)
Variance = (1 ÷ n) × Σi=18 (xi – x)2 = 9.25
=> 9.25 = (1 ÷ 8) [(-3)2 + (-2)2 + 12 + 32 + 32 + 42 + x2 + y2 – 2 × 9 × (x + y) + 2 × 92]
=> 9.25 = (1 ÷ 8) [9 + 4 + 1 + 9 + 9 + 16 + x2 + y2 – 18 × 12 + 162]
=> 9.25 = (1 ÷ 8) [48 + x2 + y2 – 216 + 162]
=> 9.25 = (1 ÷ 8) [x2 + y2 – 6]
=> x2 + y2 – 6 = 74
=> x2 + y2 = 80 ………………... (2)
From equation (1), we get
x2 + y2 + 2xy = 144 ........... (3)
From equation (2) and (3), we get
80 + 2xy = 144
=> 2xy = 64 ...................... (4)
Subtracting equation (4) from (2), we get
x 2 + y2 – 2xy = 80 – 64 = 16
=> (x – y)2 = 16
=> x – y = ± 4 ............... (5)
Therefore, from equation (1) and (5), we get
x = 8 and y = 4, when x – y = 4
x = 4 and y = 8, when x – y = –4
Thus, the remaining observations are 4 and 8.