Question Details

The value of the integral 0 π 4 log e ( 1 + tan x ) d x is:  

Options

A

π 12 log e 4

B

π 12 log e 2

C

π 16 log e 4

D

π 16 log e 2

Show Answer

Correct Answer :

Option C

π 16 log e 4

Solution :

The correct answer is:
π 16 log e 4

Step-by-step Derivation and Explanation:

Let the given definite integral be denoted by I:
I = 0 π 4 log e ( 1 + tan x ) d x --- (Equation 1)

To solve this, we can apply the properties of definite integrals. Specifically, we use the property:
a b f ( x ) d x = a b f ( a + b - x ) d x

Here, the lower limit a is 0 and the upper limit b is π4. Therefore, we substitute x with π4-x:
I = 0 π 4 log e 1 + tan π 4 - x d x

Using the trigonometric identity for the tangent of a difference:
tan ( A - B ) = tan A - tan B 1 + tan A tan B

Since tanπ4=1, we have:
tan π 4 - x = 1 - tan x 1 + tan x

Substitute this back into the expression for I:
I = 0 π 4 log e 1 + 1 - tan x 1 + tan x d x

Simplify the term inside the logarithm:
1 + 1 - tan x 1 + tan x = ( 1 + tan x ) + ( 1 - tan x ) 1 + tan x = 2 1 + tan x

Now substitute this back into the integral:
I = 0 π 4 log e 2 1 + tan x d x --- (Equation 2)

Using logarithmic properties (logeAB=logeA-logeB), we get:
I = 0 π 4 log e 2 - log e ( 1 + tan x ) d x

Splitting the integral:
I = 0 π 4 log e 2 d x - 0 π 4 log e ( 1 + tan x ) d x

Notice that the second integral is our original integral I:
I = log e 2 0 π 4 1 d x - I

Add I to both sides:
2 I = log e 2 x 0 π 4

Evaluate the limits:
2 I = log e 2 π 4 - 0
2 I = π 4 log e 2

Divide by 2 to find I:
I = π 8 log e 2

To write the answer in the form of the given options, we can rewrite loge2 using the property klogex=loge(xk):
I = π 16 × 2 log e 2
I = π 16 log e 22
I = π 16 log e 4

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