9514 1404 393
Answer:
15
Step-by-step explanation:
||3v|| = ||(-9, 12)|| = √((-9)² +12²) = √225
||3v|| = 15
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<em>Additional comment</em>
The (3, 4, 5) Pythagorean triple shows up in numerous algebra, geometry, and trigonometry problems. This is one of them. Once you identify the magnitude of v as the hypotenuse of a triangle with legs 3 and 4, you know immediately that ||v|| = 5 and ||3v|| = 15.
