Question

Give an example of a real number t for which cos t is negative and sin t is negative.

Answer 1

Anything between 3.14 and 4.71.

Answer 2

check your Unit Circle
notice the quadrants in radians where cosine and sine are negative

Answer 3

Yeah, and my unit circle says both sin and cos of any number are negative in quadrant 3, which lies between pi (3.14) and 3*pi/2=4.71.