Answer:

Step-by-step explanation:






Using the unit circle, you can visualise the secant as follows:
- Draw the vertical line passing trough (1,0)
- Extend the radius until it meets this line at point P
- The length of OP is the secant of your angle.
Since -90 means that the radius points downwards, it means that the radius is vertical as well, and thus it never meets the vertical line through (1,0).
Thus, the secant is not defined at -90.
In fact, if we switch back to the function notation, we can confirm this claim, since we have

which is undefined.
The missing reason in step 8 is the substitution property.
<h3>What is substitution property?</h3>
It should be noted that according to substitution property, if x = y, then x can be substituted for y in any equation.
When a variable is equal to another variable, then the variables can be interchangeable.
In this case, the missing reason in step 8 is the substitution property.
Learn more about substitution property on:
brainly.com/question/12289041
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Answer:
p+5+7=p+12
p/(p+12) ×900=120
900p=120(p+12)
90p=12(p+12)
90p=12p+144
90p-12p=144
78p=144
p=144/78
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