a. 437 x (5+4)
C1: 437 x 9 = 3 933
C2: 437 x (5+4) = 437 x 5 + 437 x 4
= 2 185 + 1 748 = 3 933
b.1 375 x (3+7)
C1: 1 375 x 10 = 13 750
C2: 1 375 x (3+7) = 1 375 x 3 + 1 375 x 7
= 4 125 + 9 625 = 13 750
c. 605 x (3+6)=
C1:605 x 9 = 605 x ( 10 -1 ) = 605 x 10 - 605 x 1 = 6 050 - 605 = 5 445
C2: 605 x ( 3+6) = 605 x 3 + 605 x 6
= 1 815 + 3 630 = 5 445
d. 4 007 x (2+8)
C1: 4 007 x 10 = 40 070
C2: 4007 x (2+8) = 4 007 x 2 + 4 007 x 8
= 8 014 + 32 056 = 40 070