If a unit vector is represented by 0.5î + 0.8ĵ

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If a unit vector is represented by 0.5î + 0.8ĵ + ck̂, the value of c is

1. 1
2. √0.11
3. √0.01
4. 0.39

Correct Option: B

r̂ = 0.5î + 0.8ĵ + ck̂
| r̂ | = 1 = √(0.5)² + (0.8)² + c²
(0.5)² + (0.8)² + c² = 1
c² = 0.11 ⇒ c = √0.11