1.
The position vectors of vertices of a ABC\triangle ABC are 4i^2j^4\hat{i} - 2\hat{j}, i^+4j^3k^\hat{i} + 4\hat{j} - 3\hat{k} and i^+5j^+k^-\hat{i} + 5\hat{j} + \hat{k} respectively, then ABC\angle ABC is equal to