Assertion: When I P+ Q I = I P- QI, then P must be perpendicular to Q. Reason: The relation will hold even when Q is a null vector.

The assertion is true and reason is false.

Explanation:

Assertion: I P+ Q I = I P- QI, then P must be perpendicular to Q.

Reason : The relation will hold even when Q is a null vector.

Now

So, P and Q are perpendicular to each other.

So, the assertion is true.

Reason is false.

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one "light year" is the distance that light travels, through vacuum, in one julian year.   that's 365.25 days, of 24 hours each.