(1-1) 12×(31+29)
=12× A +12× B
A 是多少?
(1-2) 12×(31+29)
B 是多少?
= A ×10+ B ×10
=( A - B )×321
=51× A - B ×49
=18× A + B ×68
=87×( A + B )
=151×( A - B )
=68×( A + B )
= C
C 是多少?
=( A - B )× C
= A ×( B + C )