RRB Group D 27-Sep-2018 Shift 1 Solved Paper

© examsiri.com

Question : 39 of 100

Marks: +1, -0

If

a

is positive and

a^{2}+{1}/{a^{2}}=7,

then

a^{3}+{1}/{a^{3}}=

21
$3 \√{7}$
18
$7 \√{7}$

Solution:

a^{2}+{1}/{a^{2}}=7

....(i)

(a+{1}/{a})^{3}=a^{3}+{1}/{a^{3}}

+3(a+{1}/{a})

....(ii)

(a+{1}/{a})^{2}=a^{2}+{1}/{a^{2}}+2,

From (i),

(a+{1}/{a})^{2}=9, a+{1}/{a}=3

Putting in equation (ii),

27=a^{3}+{1}/{a^{3}}+3 ×3

a^{3}+{1}/{a^{3}}=18

© examsiri.com

Go to Question:

Prev Question

Next Question