Bài 1:

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

$=\left(a+b+c\right)^2+\left(a+b-c\right)^2+\left(a-b-c\right)^2+\left(a-b+c\right)^2$

$=\left(a+b\right)^2+2c\left(a+b\right)+c^2+\left(a+b\right)^2-2c\left(a+b\right)+c^2+\left(a-b\right)^2-2c\left(a-b\right)+c^2+\left(a-b\right)^2+2c\left(a-b\right)+c^2$$=2\left(a+b\right)^2+2c^2+2\left(a-b\right)^2+2c^2$

$=2\left[\left(a+b\right)^2+\left(a-b\right)^2\right]+4c^2$

$=4\left(a^2+b^2+c^2\right)$

Bài 2:

$\left(a+b-c\right)^2+\left(a-b+c\right)^2-2.\left(b-c\right)^2$

$=\left(a+b-c\right)^2-\left(b-c\right)^2+\left(a-b+c\right)^2-\left(b-c\right)^2$

$=\left(a+b-c+b-c\right)\left(a+b-c-b+c\right)\left(a-b+c-b+c\right)\left(a-b+c+b-c\right)$$=a^2+a^2$

$=2a^2$

Bài 3 cũng tương tự nha bạn.

Chúc bạn học tốt!