a)1+2+3+4+5+6+7+8+9
=(1+9)+(2+8)+(3+7)+(4+6)+5
=10+10+10+10+5
=10x4+5=45
b)8x3+8x7
=8x(3+7)
=8x10=80
c)8x172-8x72
=8x(172-72)
=8x100=800
a. 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 +9
= (1 + 9) + (2 + 8) + (3 + 7) + ( 4 + 6) + 5
= 10 + 10 + 10 + 10 + 5
= 45