Two adjacent sides of a parallelogram ABCD are given by
$$\overrightarrow{AB} = 2\hat{i} + 10\hat{j} + 11\hat{k}$$ and $$\overrightarrow{AD} = -\hat{i} + 2\hat{j} + 2\hat{k}$$
The side AD is rotated by an acute angle $$\alpha$$ in the plane of the parallelogram so that AD becomes AD'. If AD' makes a right angle with the side AB, then the cosine of the angle $$\alpha$$ is given by
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