7. กราฟของสมการ \( \mathrm{mx}+\mathrm{ny}=7 \) และ \( \mathrm{nx}+\mathrm{my}=8 \) ตัดกันที่จุด \( (1,2) \) ค่าของ \( \mathrm{m}+\mathrm{n} \) ตรงกับข้อใด? ก. 2 ข. 3 ค. 4 ง. 5

\( 1 \leftarrow \quad \) Determine whether \( B \) is the inverse of \( A \). \( A=\left[\begin{array}{rr}3 & -7 \\ -2 & 5\end{array}\right], \quad B=\left[\begin{array}{ll}5 & 7 \\ 2 & 3\end{array}\right] \)

Three-fifths of Jesse Black's inventory was destroyed by fire. He sold the remaining part, which was slightly damaged, for two-thirds of its value and received \( \$ 3400 \). (a) What was the value of the destroyed part of the inventory? (b) What was the value of the inventory before the fire? (a) The value is \( \$ \square \). (Round to the nearest cent as needed.)

\begin{tabular}{l|l}\( \begin{array}{l}\text { Simplify the expression so that no negative exponents } \\ \text { appear in the final result. Assume all variables represent } \\ \text { nonzero real numbers. }\end{array} \) & \( \begin{array}{l}\left(4 a^{-2}\right)^{4}\left(a^{4}\right)^{-3}=\square \\ \\ \left(4 a^{-2}\right)^{4}\left(a^{4}\right)^{-3}\end{array} \) \\ exponents only.) \end{tabular}

a) \( x+y=2 \) b) \( x+x-y=4+y \)

Résoudre dans \( \mathbb{C}: \mathrm{z}^{7}=1 \)