Lời giải:
a.
$=(\sin ^2a+\cos ^2a)^2=1^2=1$
b.
$=\tan ^2a(1-\sin ^2a)=\tan ^2a.\cos ^2a=\frac{\sin ^2a}{\cos ^2a}.\cos ^2a=\sin ^2a$
c.
$=\cos ^2a(1+\tan ^2a)=\cos ^2a(1+\frac{\sin ^2a}{\cos ^2a})$
$=\cos ^2a.\frac{\sin ^2a+\cos ^2a}{\cos ^2a}=\sin ^2a+\cos ^2a=1$
d.
$=\tan ^2a(\cos ^2a+1-1)=\tan ^2a\cos ^2a=\frac{\sin ^2a}{\cos ^2a}.\cos ^2a=\sin ^2a$