The cofunction identity for cotangent Theta equals tangent of π halves minus Theta. This is one of our identity formulas that we need to know the cotangent Theta equal tangent of π halves minus Theta. If you're struggling with this, think about in the right triangle. If we know Theta, we know that the other angle has to be π halves minus Theta because they're complementary angles. If we think about the cotangent being the adjacent side over the opposite side, then when we take the complementary angle, it's got to be the opposite side over the adjacent. And so if cotangent Theta is 9 sevenths then tangent of π halves minus Theta has to also be 9 sevenths.