a. Positive (`100^@` is in the 2nd quadrant - `sin` is positive)

b. Positive (`-15^@` is in the fourth quadrant, since negative angles are measured clockwise. Since `cos` is positive in the 4th quadrant, so `sec` will be also.)

c. Negative (`188^@` is in the 3rd quadrant, and `cos` is negative there).

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