**18. Correct. The
answer is true**

(a) If the area to the right of t_{1
}=0.05, then the area to its left is 0.95 and t_{1} represents the
95^{th} percentile, which, according to the table of the
t-distribution, is equal to 1.83.

(b) If the area to the left of -t_{1}
and to the right of t_{1 }=0.05, then the area to the right of t_{1}
is 0.025 by symmetry. Thus the area to the left of t_{1} is 0.975 and t_{1}
represents 97.5^{th} percentile, or 2.26.

(c) If the area between -t_{1 }and
t_{1 }is 0.99, then the area of the complement if 0.01 and the area to
the right of t_{1} is 0.005. Thus t_{0.995} =3.25.

(d) If the area to the left of -t_{1}
is 0.01, then by symmetry the area to the right of t_{1} is 0.01. Since
t_{0.99}=2.82, the value of t for which the area to the left is 0.01 is
-2.82.

(e) If the area to the left of t_{1}
is 0.90, then t_{0.90}=1.38.