Answer
$$ y' =2t \,sech^2(t^2+1)
.$$
Work Step by Step
Since $ y=\tanh (t^2+1)$, then the derivative $ y'$, using the chain rule, is given by
$$ y'= sech^2(t^2+1) ( t^2+1)'=2t \,sech^2(t^2+1)
.$$
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