Final answer:
To apply the cofunction theorem to the expression tan(90° - x°), you would fill in the blank with cot(x°), which shows the relationship between the tangent of a complementary angle and the cotangent of the angle.
Step-by-step explanation:
The student is asking about the cofunction theorem in trigonometry, which deals with the relationships between the trigonometric functions of complementary angles. Specifically, they need to fill in a blank to make a true statement involving the tangent and cotangent functions.
To solve this, we recall that trigonometric functions of complementary angles are related as follows: tan(90° - x°) = cot(x°). So, the expression to complete the statement in the question is cot(x°). This is a direct application of the cofunction theorem that states tan(90° - θ) = cot(θ) and vice versa.