M=a^3+b^3+c^3-3abc/(a-b)^3+(b-c)^3+(c-a)^3

nè ban

\(\left(a+b\right)^3+\left(b+c\right)^3+\left(c+a\right)^3-3\left(a-b\right)\left(b-c\right)\left(c-a\right)\)

Đặt a+b=x ; b+c=y; c+a=z ta có:

\(x^3+y^3+z^3-3xyz\)

=\(\left(x+y\right)^3-3x^2y-3xy^2+z^3-3xyz\)

=\(\left[\left(x+y\right)^3+z^3\right]-\left(3x^2y+3xy^2+3xyz\right)\)

\(=\left(x+y+z\right)\left[\left(x+y\right)^2-z\left(x+y\right)+z^2\right]-3xy\left(x+y+z\right)\)

\(=\left(x+y+z\right)\left(x^2+2xy+y^2-xz-yz+z^2-3xy\right)\)

\(=\left(x+y+z\right)\left(a^2+b^2+c^2-ab-ac-bc\right)\)

xong thay vào

Đặt a+b = x ; b+c = y ; c+a = z

=> H = x^3 +y^3 +z^3 -3.x.y.z

         = [x+y]^3 -3.x^2.y -3.x.y^2+ z^3 - 3.x.y.z

          = {[x+y]^3+z^3} -3.x.y[x+y+z]

          = [x+y+z].{[x+y]^2-[x+y].z+z^2} +3.x.y[x+y+z]

          = [x+y+z] . [x^2+y^2+2.x.y-x.z-y.z+z^2+3.x.y]

           = [x+y+z]. [x^2+y^2+z^2-xy-y.z-x.z]

           = [a+b+b+c+c+a]. {[a+b]^2+[b+c]^2+[c+a]^2-[a+b].[b+c]-[a+b].[a+c] - [b+c].[c+a]}

            = 2.[a+b+c] .[a^2+b^2 +b^2 +c^2 +c^2 +a^2 +2.ab.+2.bc+2.ac-ab-b^2-ac-bc-a^2-ab-ac-bc-bc-c^2-ab-ac]

            = 2.[a+b+c].[a^2+b^2+c^2-ab-ac-bc]

rút gọn a

(-a-b+c)-(-a-b-c)

Áp dụng hằng đẳng thức dưới dạng 

\(x^3+y^3=\left(x+y\right)^3-3xy\left(x+y\right)\)

\(\left(a+b+c\right)^3+\left(a-b-c\right)^3=\left(2a\right)^3-3\left(a+b+c\right)\left(a-b-c\right).2a\)

\(\left(b-c-a\right)^3+\left(c-a-b\right)^3=\left(-2a\right)^3-3\left(b-c-a\right)\left(c-a-b\right).\left(-2a\right)\)

\(\Rightarrow\left(a+b+c\right)^3+\left(a-b-c\right)^3+\left(b-c-a\right)^3+\left(c-a-b\right)^3\)

\(=\left(2\right)^3+\left(-2a\right)^3-6a\left[a+\left(b+c\right)\right]\left[a-\left(b+c\right)\right]+6a\left[-a+\left(b-c\right)\right]\left[-a-\left(b-c\right)\right]\)

\(=-6a\left\{a^2-\left(b+c\right)^2-\left[\left(-a\right)^2-\left(b-c\right)^2\right]\right\}\)

\(=-6a\left\{a^2-a^2+\left(b-c\right)^2-\left(b+c\right)^2\right\}\)

\(=-6a\left[b-c+b+c\right]\left[b-c-\left(b+c\right)\right]=-6a.2b.\left(-2c\right)\)

\(=24abc\)

ừ chie cần k vaod chữ đúng thôi

a,Đặt a+b-c=x, c+a-b=y, b+c-a=z

=>x+y+z=a+b-c+c+a-b+b+c-a=a+b+c

Ta có hằng đẳng thức:

(x+y+z)^3-3x-3y-3z=3(x+y)(x+z)(y+z)

=>(a+b+c)^3-(b+c-a)^3-(a+c-b)^3-(a+b-c)^3=(x+y+z)^3-x^3-y^3-z^3

=3(x+y)(x+z)(y+z)

=3(a+b-c+c+a-b)(c+a-b+b+c-a)(b+c-a+a+b-c)

=3.2a.2b.2c

=24abc

sao ma kho du day ban..minh bo tay bo chan lun oy oy oy

xin loi minh khong the giup ban duoc

mk chưa hok tới lớp 8