⑴ab-(a-2)(b-2)
=ab-(ab-2a-2b+4)
=2a+2b-4.
⑵2a+2b-4=1/2ab
ab=4a+4b-8,
∴(a-4)(b-4)
=ab-4a-4b+16
=(4a+4b-8)-4a-4b+16
=8.