A line $AB$ in three-dimensional space makes angles ${45^ \circ }$ and ${120^ \circ }$ with the positive $x$-axis and the positive $y$-axis respectively. If $AB$ makes an acute angle $\theta $ with the positive $z$-axis, then $\theta $ equals :
${45^ \circ }$
${60^ \circ }$
${75^ \circ }$
${30^ \circ }$
Solution
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