a) 72×27+27+28+(39×93–39×39):54
=1944+27+28+(3627–1521):54
=1971+28+2106:54
=1971+28+39
=1999+39
=2038
b) 1997×109–199700–1997×7+1997
=217673–199700–13979+1997
=17973–13979+1997
=3994+1997
=5991
3) a×b=30 si a×c=18; a(b+c) si a(b–c)
a(b+c)
=a×b+a×c
=30+18
=48
si
a(b–c)
=a×b–a×c
=30–18
=12
4) 3a+2b+4c=49 si b+2c=160
ab+2ac=a(b+2c)
3a+2(b+2c)=494
3a+2×160=494
3a+320=494
3a=494–320
3a=174
a=174:3
a=58
a(b+2c)
=58×160
=9280
5)
9a+6b+12c=690 si b+2c=40
ab+2ac si a =?
9a+6(b+2c)=690
9a+6×40=690
9a+240=690
9a=690–240
9a=450
a=450:9
a=50
a(b+2c)=50×40=200
