Trig help (sq. roots?!)

P = (x,y) is the point on the unit circle that corresponds to a real number t. Find the exact values of the six trigonometric functions of t.

I know the formulas but I have no idea how to simplify the square roots.
P = (-2/5 , √21/5)

sin t = √21/5
cos t = -2/5
tan t = -(√21/2)

Ok, this is where I have no idea how to simplify this square root so that the integer is in the denominator. I was absent in class today so I missed out a lot.
csc t = 1/(√21/5)

My textbook says the answer is (5√21)/21
Why? How??

sec t = -5/2

cot = -(2/5)/(√21/5)
Textbook says simplified is -(2√21)/21

I have no idea how.... Please someone.

To simplify a fraction with a root in the denominator, you would use a procedure called "rationalize the denominator". This is where you would multiply the numerator and denominator by 1 in the form of root(some number)/root(some number)

1/(√21/5) = 5/√21 = (5/√21)*(√21/√21) = (5√21)/21

-(2/5)/(√21/5) = -(2/5)*(5/√21) = -2/√21 = (-2/√21)*(√21/√21) = -(2√21)/21