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# Vector help please! watch

1. Relative to an origin O, the position vectors of the points P and Q are given by OP = 3i + j + 4k and OQ = i + xj - 2k.
Find the values of x for which the magnitude of PQ is 7.
How do I do this?
2. (Original post by Rienet)
Relative to an origin O, the position vectors of the points P and Q are given by OP = 3i + j + 4k and OQ = i + xj - 2k.
Find the values of x for which the magnitude of PQ is 7.
How do I do this?
The position vectors essentially tell us the coordinates of points P and Q. From there, we can calculate the vector PQ.

The magnitude is calculated via Pythagoras' Theorem and so we can equate this to 7 and rearrange for x.

Get back to me if you get stuck.

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