Đáp án:
Vậy $A=-45a-90b$
Giải thích các bước giải:
$A=15(a+2b)^2-3(a+2b)(a+2b+19)+6(2a+4b)(1-a-2b)$
Đặt $n=a+2b$
$=>A=15n^2-3n(n+19)+6(2a+4b)(1-a-2b)$ (Thay n vào thôi)
$=>A=15n^2-3n(n+19)+6[2(a+2b)][1-(a+2b)]$
[vì $2a+4b=2a+2.2b=2(a+2b)$]
$=>A=15n^2-3n(n+19)+6.2n(1-n)$
$=>A=15n^2-(3n.n+3n.19)+12n(1-n)$
$=>A=15n^2-(3n^2+57n)+12n.1-12n.n$
$=>A=15n^2-3n^2-57n+12n-12n^2$
$=>A=(15n^2-3n^2-12n^2)+(12n-57n)$
$=>A=12n-57n$
$=>A=(12-57)n$
$=>A=-45n$
$=>A=-45(a+2b)$
$=>A=-45a+-45.2b$
$=>A=-45a+(-90b)$
$=>A=-45a-90b$
Vậy $A=-45a-90b$
