Question

[Huỳnh Thị Bích Tuyền]

1/ CMR : \(\left(a+b\right)^2=\left(a-b\right)^2+4ab\)

\(\left(a-b\right)^2=\left(a+b\right)^2-4ab\)

2/ Tính : 

\(\left(a+b+c\right)^2\)

Answer 1

[Minh Triều]

1)VP=(a-b)²+4ab=a²-2ab+b²+4ab

   =a²+2ab+b²=(a+b)²=VT

Vậy (a+b)²=(a-b)²+4ab

VP = (a+b)²-4ab=a²+2ab+b²-4ab

     =a²-2ab+b²=(a-b)²=VT

Vậy (a-b)²=(a+b)²-4ab

2)(a+b+c)²=[(a+b)+c]²=(a+b)²+2(a+b)c+c²=(a²+2ab+b²)+2ac+2bc+c²

                                                                                    =a²+b²+c²+2ab+2ac+2bc