- Đồng vị thứ nhất có:A₁=35+44=79

- Đồng vị thứ 2 có: A₂=79+2=81

\(\overline{A}=\dfrac{27.79+23.81}{27+23}=79,92\)

3(12+x)=7(x-4)

\(\Leftrightarrow\)36+3x=7x-28

\(\Leftrightarrow\)7x-3x=36+28

\(\Leftrightarrow\)4x=64

\(\Leftrightarrow\)x=16

2,6 km² + 5,87 ha + 400000 m² + 1300 m²

= 2,6 km² + 0,0587 km² + 0,4 km² + 0,0013 km²

= 3,06 km²

\(\Leftrightarrow\)4x²-8x+4=9x²+42x+49

\(\Leftrightarrow\)5x²+50x+45=0

\(\Leftrightarrow\)x²+10x+9=0

\(\Leftrightarrow\)x²+9x+x+9=0

\(\Leftrightarrow\)x(x+9)+(x+9)=0

\(\Leftrightarrow\)(x+1)(x+9)=0

\(\Leftrightarrow\)\(\left[{}\begin{matrix}x+1=0\\x+9=0\end{matrix}\right.\)

\(\Leftrightarrow\)\(\left[{}\begin{matrix}x=-1\\x=-9\end{matrix}\right.\)