# The vectors `veca` and `vecb` are unit vectors such that a· b=1/2. Determine the value of j such that the vectors a+b and 2a+jb are perpendicular.

You need to remember that the cross product of perpendicular vectors is zero such that:

`(abari + bbarj)(cbari + dbarj) = ac + bd = 0`

You need to form the vectors `barv_1`  and `barv_2`  substituting  bar a for unit vector bari and bar b for unit vector bar j such that:

`barv_1 = bara+barb`

`barv_2 = 2bara + jbarb`

You need to evaluate cross product `barv_1*barv_2`  such that:

`barv_1*barv_2 = 0`

Multiplying the coefficients of like unit vectors yields:

`2*1 + 1*j = 0`

j = -2

Hence, evaluating the value of j under given conditions yields j=-2.

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