The $t$ -table has entries for areas of 0.10,0.05,0.025,0.01 and $0.005 .$ In contrast, the $x^{2}$ -table has entries for those areas and for $0.995,0.99,0.975,0.95,$ and $0.90 .$ Explain why the $t$ -values corresponding to these additional areas can be obtained from the existing $t$ -table but must be provided explicitly in the $x^{2}$ -table.