Cosine of 90°

You may be somewhat fuzzy about how the cosine function behaves. Rather than memorize abstract stuff, I prefer to visualize the unit circle with its radius projected onto the x-axis.

From the picture,

the cosine of 0°   =   1.0,
the cosine of 30°   =   0.866,
the cosine of 45°   =   0.707,
the cosine of 60°   =   0.500,
the cosine of 90°   =   0.0.

Recall that:

u · v = |u| |v| cos θ