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Question
Find the unit vector in the direction of the sum of the vectors `vec"a" = 2hat"i" - hat"j" + hat"k"` and `vec"b" = 2hat"j" + hat"k"`.
Solution
Given that `vec"a" = 2hat"i" - hat"j" + hat"k"` and `vec"b" = 2hat"j" + hat"k"`.
`vec"a" + vec"b" = (2hat"i" - hat"j" + hat"k") + (2hat"j" + hat"k")`
= `2hat"i" + hat"j" + 2hat"k"`
∴ Unit vector in the direction of `vec"a" + vec"b" = (vec"a" + vec"b")/|vec"a" + vec"b"|`
= `(2hat"i" + hat"j" + 2hat"k")/sqrt((2)^2 + (1)^2 + (2)^2)`
= `(2hat"i" + hat"j" + 2hat"k")/sqrt(4 + 1 + 4)`
= `(2hat"i" + hat"j" + 2hat"k")/sqrt(9)`
= `(2hat"i" + hat"j" + 2hat"k")/3`
= `2/3hat"i" + 1/3hat"j" + 2/3hat"k"`
Hence, the required unit vector is `2/3hat"i" + 1/3hat"j" + 2/3hat"k"`.
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