Find the value of x such that a and b are orthogonal,

where a = 2i + xj, and b = -5i + xj.

If two vectors are orthogonal, the angle between them is 90° , making the cosine of the angle between them 0.

Hence, we proceed by finding the value of x which makes the cosine 0:

. Here this becomes

. We need to solve only the numerator:

Hence