3. (х-3)(х+4) = х(х+1)-12
х²+4х-3х-12 = х²+х-12
х-12 = х-12
-12 = -12
4. а). (а+8)²-(а-4)(а+4) = а²+16а+64-(а-4)(а+4) = а²+16а+64-а²-4а+4а+16 = 16а+64+16 = 16а+80 при а=0,6 16·0,6+80 = 9,6+80 = 89,6
б). 1 5/19×(6-3 5/8) = 24/19×(48/8-29/8) = 24/19×19/8 = 3