Answer
(a) -$\hat{j}$
(b) 0
Work Step by Step
(a) $\hat{i}\times (\hat{i}\times \hat{j})= \hat{i}\times (\hat{k})= -\hat{j}$
(b) $(\hat{i}\times \hat{j})\times \hat{k}= (\hat{k})\times \hat{k}= 0$
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