13.
\(a-b=12\)
\(a\cdot c=20\)
\(b\cdot c=-28\)
\(\Rightarrow c=?\)
14.
\(x-y=12\)
\(y+z=7\)
\(x-z=13\)
\(x=?\)
15.
\(x-y=14\)
\(x+z=10\)
\(y-z=-6\)
\(\Rightarrow x=?\)
16.
\(3x+y+2z=18\)
\(2x+4y-3z=20\)
\(x+y-3z=12\)
\(\Rightarrow x-y+z=?\)
17.
\(\left.\begin{aligned}\samefrac{x\cdot z}y=\samefrac{21}2\\\samefrac{y\cdot z}x=\samefrac{25}{14}\\\samefrac{x\cdot y}z=\samefrac{16}{15}\end{aligned}\right\}\)
\(\Rightarrow x\cdot y\cdot z=?\)
18.
\(a,b,c\in\mathbb R^+\)
\(a\cdot b=9\cdot c\)
\(b\cdot c=16\cdot a\)
\(\Rightarrow b=?\)
19.
\(7m-2n=38\)
\(m+4n=14\)
\(\Rightarrow m^2-n^2=?\)
20.
\(\samefrac6b-\samefrac{a\cdot c-x^2}{a\cdot x}=\samefrac{7x-b\cdot c}{b\cdot x}\)
\(\Rightarrow x=?\)
21.
\(a\neq b\)
\(3a+\samefrac5a=3b+\samefrac5b\)
\(\Rightarrow a\cdot b=?\)
22.
\(x>0\)
\(y>0\)
\(\samefrac{x+y}{15}=\samefrac{x-y}5\)
\(x^2-y^2=243\)
\(\Rightarrow x=?\)
23.
\(x,y,z\in\mathbb R^+\)
\(\left.\begin{aligned}x^3\cdot y^2=4z\\y^3\cdot z=2x\\z^4\cdot x^2=32y\end{aligned}\right\}\)
\(\Rightarrow x\cdot y\cdot z=?\)