Deceptive Integral

Calculus Level 3

Evaluate 0 1 4 x + 1 2 x + 1 d x . \int_0^1 \dfrac{4^x + 1}{2^x + 1} \, dx .

Note: Just choose a good change of variable


The answer is 1.272.

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1 solution

Mark Hennings
Dec 1, 2018

We have I = 0 1 ( 2 x + 1 ) 2 2 × 2 x 2 x + 1 = 0 1 ( 2 x + 1 2 2 x 2 x + 1 ) d x = 1 ln 2 + 1 2 0 1 1 2 x + 1 2 x d x I \; = \; \int_0^1 \frac{(2^x+1)^2 - 2 \times 2^x}{2^x+1} \; = \; \int_0^1 \left(2^x + 1 - 2 \frac{2^x}{2^x+1}\right)\,dx \; = \; \frac{1}{\ln2} + 1 - 2\int_0^1 \frac{1}{2^x+1}\,2^x\,dx Using the substitution y = 2 x y = 2^x , we obtain I = 1 ln 2 + 1 2 ln 2 1 2 1 y + 1 d y = 1 ln 2 + 1 2 ln 2 [ ln ( y + 1 ) ] 1 2 = 1 ln 9 8 ln 2 = 1.272770039 I \; = \; \frac{1}{\ln2} + 1 - \frac{2}{\ln2}\int_1^2\frac{1}{y+1}\,dy \; = \; \frac{1}{\ln2} + 1 - \frac{2}{\ln2}\Big[\ln(y+1)\Big]_1^2 \; = \; \frac{1 - \ln\frac98}{\ln2} \; = \; \boxed{1.272770039}

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