Question Details

Given x+1x=5, find x3+1x3

Options

A

100

B

110

C

120

D

125

Show Answer

Correct Answer :

Option B

110

Solution :

The correct option is 110.

To find the value of x3+1x3, we can use the algebraic identity for the cube of a sum:

(a+b)3=a3+b3+3ab(a+b)

Let a=x and b=1x. Substituting these into the identity gives:

(x+1x)3=x3+1x3+3·x·1x(x+1x)

Simplifying the product term x·1x=1, the equation becomes:

(x+1x)3=x3+1x3+3(x+1x)

We are given that x+1x=5. Substitute this value into the equation:

53=x3+1x3+3(5)

Now, calculate the numerical values:
53=125
3·5=15

Substitute these values back into the equation:

125=x3+1x3+15

Subtract 15 from both sides to isolate x3+1x3:

x3+1x3=125-15

x3+1x3=110

Thus, the required value is 110.

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