1.
Let a=2i^7j^+5k^\vec{a} = 2\hat{i} - 7\hat{j} + 5\hat{k}, b=i^+k^\vec{b} = \hat{i} + \hat{k} and c=i^+2j^3k^\vec{c} = \hat{i} + 2\hat{j} - 3\hat{k} be three given vectors. If r\vec{r} is a vector such that r×a=c×a\vec{r} \times \vec{a} = \vec{c} \times \vec{a} and rb=0\vec{r} \cdot \vec{b} = 0, then r|\vec{r}| is equal to