cả hai x = 0

1x + 2x =0 

x.(1 + 2) = 0

x.3 = 0

   x = 0 : 3

   x = 0

có sai đề ko bạn

đặt x+1/3=k

      y+3/4=k

=>x=1/3k

    y=3/4k

ta có:xy=3

     1/3k*3/4k=3

     1/4k        =3

      k            =12

với k=12=>x=35/3

                   y=45/4

\(4^{x+2}\cdot5\cdot4^x=1280\)

\(\Leftrightarrow4^x\cdot16\cdot5\cdot4^x=1280\)

\(\Leftrightarrow4^{2x}\cdot80=1280\)

\(\Leftrightarrow4^{2x}=16\)

\(\Leftrightarrow4^{2x}=4^2\)

\(\Leftrightarrow2x=2\)

\(\Leftrightarrow x=1\)

vậy........

a) \(S=5+5^2+5^3+5^4+.......+5^{96}\)

\(\Rightarrow5S=5^2+5^3+5^4+5^5+.........+5^{97}\)

\(\Rightarrow5S-S=5^{97}-5\)

\(\Rightarrow4S=5^{97}-5\)\(\Rightarrow S=\frac{5^{97}-5}{4}\)

b) \(S=5+5^2+5^3+5^4+..........+5^{96}\)

\(=\left(5+5^4\right)+\left(5^2+5^5\right)+\left(5^3+5^6\right)+.....+\left(5^{93}+5^{96}\right)\)

\(=5\left(1+5^3\right)+5^2.\left(1+5^3\right)+5^3.\left(1+5^3\right)+......+5^{93}.\left(1+5^3\right)\)

\(=5\left(1+125\right)+5^2.\left(1+125\right)+5^3.\left(1+125\right)+......+5^{93}.\left(1+5^3\right)\)

\(=5.126+5^2.126+5^3.126+......+5^{93}.126\)

\(=126.\left(5+5^2+5^3+.........+5^{93}\right)⋮126\)( đpcm )

\(B=x^2-3x\)

\(=x^2-2\cdot\frac{3}{2}\cdot x+\frac{9}{4}-\frac{9}{4}\)

\(=\left(x-\frac{3}{2}\right)^2-\frac{9}{4}\ge-\frac{9}{4}\)

dấu = xảy ra

\(\Leftrightarrow x-\frac{3}{2}=0\)

\(\Leftrightarrow x=\frac{3}{2}\)

vậy.........

27x³-y³=(3x-y)(9x²+3y+y²)

=>27x³-y³:3x-y=9x²+3y+y²

                                                   Hok ^Tốt