Answer
$$ du = \frac{dt}{(t+1)^2} .$$
Work Step by Step
Recall the quotient rule: $(\dfrac{u}{v})'=\dfrac{vu'-uv'}{v^2}$
$$ u= \frac{t}{t+1}\Longrightarrow du = \frac{(t+1)(1)-t(1)}{(t+1)^2} d t=\frac{dt}{(t+1)^2} .$$
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