5)
You have the right idea. You need to put the "reason" on the line of the result of using the reason. Your reasons are generally correct, but not in the expected place in the table.
6)
In general, in geometry, the three ideas of point, line, plane are undefined. In 6a, your answers of point and line were marked correct.
In 6b, you are expected to understand that the idea of circle is defined (the set of points in a plane equidistant from the center point). Of course, the point of the sentence, "A secant line is ..." is to define the term secant line. I agree with your answer to 6b, but apparently your teacher was looking for "secant," rather than "secant line."
8)
Here are the relationships:
- Conditional: if P, then Q.
- Converse: if Q, then P
- Inverse: if not P, then not Q
- Contrapositive: if not Q, then not P
8a. See the second attachment for possibilities.
8b. The hypothesis is the "if" part of the conditional expression.
9)
A conditional statement can be written as a biconditional only when both the conditional and the converse are true. It is generally written in the form, ...
... P if and only if Q
The given statement cannot be written as a biconditional because the converse is not true (a right triangle is not an acute triangle). A slight modification makes the converse true and makes it a valid biconditional:
... A triangle has one angle of 90° or more if and only if the triangle is not an acute triangle.