7.3 a) $$\frac{1}{cos^2t} - 1 = \frac{1-cos^2t}{cos^2t} = \frac{sin^2t}{cos^2t} = tg^2t$$.
б) $$\frac{1-sin^2t}{cos^2t} = \frac{cos^2t}{cos^2t} = 1$$.
Ответ: а) $$tg^2t$$, б) 1.