Answer:
36.9°
Step-by-step explanation:
You want to know the pitch angle of a staircase with a riser height of 6 inches and a tread depth of 8 inches.
Tangent
The tangent of an angle is the ratio of the side opposite to the side adjacent:
Tan = Opposite/Adjacent
Application
The tangent of the staircase angle to the floor will be the ratio of the riser height to the tread depth:
tan(α) = (6 in)/(8 in) = 3/4
The angle is found using the inverse tangent function.
α = arctan(3/4) ≈ 36.9°
__
Additional comment
You need to be a little bit careful here. Different authors define "tread depth" differently. For the purpose of this question, it must be defined as the horizontal distance between corresponding points on adjacent treads, as shown in the attached illustration.
Some authors define tread depth as the distance from the nosing to the riser. If that definition is used, the "run" of the step will be the tread depth less the nosing depth. The pitch angle will be the arctangent of the ratio of riser height to "run".
The tread depth of 8 inches is a little too narrow to be in compliance with some building codes. A more usual minimum is about 10 inches with a riser height of 7 1/2 inches.
