Verify the identity sec (theta) sin (theta) cot (theta) = 1

Secant = hypotenuse / adjacent

sine = opposite / hypotenuse

cotangent = adjacent / opposite

So, if we multiply all three we have a numerator of

hypotenuse * opposite * adjacent and a denominator of

adjacent * hypotenuse * opposite

Notice that "hypotenuse", "opposite", and "adjacent" appear exactly once in the numerator AND the denominator.

When numerator = denominator, then a fraction equals ONE.