Assuming the angle made by a vector refers to the angle it makes with the positive horizontal axis, take the dot product of u with the vector (1, 0), and recall that
a • b = ||a|| ||b|| cos(θ)
where θ is the angle made by the vectors a and b. Then
u • (1, 0) = ||u|| ||(1, 0)|| cos(θ)
We have
u • (1, 0) = 5×1 + 8×0 = 5
||u|| = √(5² + 8²) = √89
||(1, 0)|| = √(1² + 0²) = √1 = 1
so that
5 = √89 cos(θ)
cos(θ) = 5 / √89
θ = arccos(5 / √89) ≈ 57.99°
