Integration

(1) Let I=\int_{1/2}^{2}{\frac{ln(t)}{1+t^{n}}}dt,
find the sign of Integral for different values of n\varepsilon N +\left\{0 \right\}

2 Answers

341
Hari Shankar ·

I = \int_{\frac{1}{2}}^2 \frac{\ln t}{1+t^n} \ dt = \int_{\frac{1}{2}}^1 \frac{\ln t}{1+t^n} \ dt + \int_{1}^2 \frac{\ln t}{1+t^n} \ dt

= \int_{\frac{1}{2}}^1 \frac{\ln t}{1+t^n} \ dt - \int_{\frac{1}{2}}^1\frac{t^{n-2} \ln t}{1+t^n} \ dt = \int_{\frac{1}{2}}^1\frac{(1-t^{n-2}) \ln t}{1+t^n} \ dt

We have ln t<0 for 0<t<1

Also for n>2 1-t^{n-2}>0 in this range and for n≤2 1-t^{n-2}<0

Thus we see that the integral is negative for n>2 and +ve for n≤2

341
Hari Shankar ·

I am unable to edit the post, it should be +ve for n=0,1 and -ve otherwise

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