a,=(2a + b - 3c).(2a + b - 3c)
=4a\(^2\)+2ab-6ac+2ab+b\(^2\)-3bc-6ac-3cb+9c\(^2\)
=4a\(^2\)+b\(^2\)+9c\(^2\)+4ab
=2\(^2\).a\(^2\)+4ab+b\(^2\)+9c\(^2\)
=(2a+b)\(^2\)+9c\(^2\)( đáng lẽ chỗ này nó phải là -9c\(^2\) nhưng t ko ra đc )
b,=(a + 2b + 3c - 4d)(a + 2b + 3c - 4d)
=a\(^2\)+2ab+3ac-4ad+2ab+4b\(^2\)+6bc-8bd+3ac+6bc+9c\(^2\)-12cd-4ad-8bd-12cd+16d\(^2\)
=a\(^2\)+4b\(^2\)+9c\(^2\)+16d\(^2\)+4ab+6ac-8ad+12bc-16bd-24cd
=(a\(^2\)+4ab+4b\(^2\))+(9c\(^2\)-24cd+16d\(^2\))+6ac-8ad+12bc-16bd
=(a+2b)\(^2\)+(3c-4d)\(^2\)+2(3ac-4ad+6bc-8bd)
=(a+2b)\(^2\)+(3c-4d)\(^2\)+2[a(3c-4d)+2b(3c-4d)]
=(a+2b)\(^2\)+(3c-4d)\(^2\)+2(a+2b)(3c-4d)
khiếp bài dài nghoằng ra ý :(
