ta có
vt = a(b-c)+a(d+c) (1)
= ab - ac + ad + ac
= (ac-ac) + (ab+ad)
= 0 + a(b+d)
= a(b+d)
vp = a(b+d) (2)
(1)(2) => đpct
Ta có:
a(b - c) + a(d + c)
= a . b - a . c + a . d + a . c
= (-a . c + a . c) + a . b + a . d
= 0 + a (b + d)
= a (b + d) = a (b + d)
Vậy a(b - c) + a(d + c) = a(b + d)