The unit circle is a circle whose radius is one and center is at the origin.
According to Pythagorean Theorem,
p2 + q2 = 1
Also, the trigonometric functions cosine and sine may be defined on the unit circle.
p = cos(θ)
q = sin(θ)
Therefore, the equation p2 + q2 = 1 gives the relation
cos2(θ) + sin2(θ) = 1
Also,
tan (θ) = sin(θ)/ cos(θ) = q/p
cosec (θ) = 1/sin(θ) = 1/q
sec (θ) = 1/cos (θ) = 1/p
cot (θ) = 1/tan (θ) = p/q
