A
I − A
I
3A
(I+A)3-7A =I3+A3+3I2A+3A2I-7A
=I+A3+3A+3A2-7A
=I+A2.A+3A+3A-7A [A2=A]
=I+A2-A
=I+A-A
=I
\(\therefore\) (I+A)3-7A=I