Tìm x

1.tìm x :$\frac{x-8}{3}+\frac{x-7}{4}+\frac{x-1}{5}=4$
2.tìm x,y,z
a.$x\left(x+y+z\right)=-5,y\left(x+y+z\right)=9,z\left(x+y+z\right)=5$

b.$\frac{x\times y}{x+y}=\frac{12}{7},\frac{yz}{y+z}=-6,\frac{zx}{zx}=-4$

3.tĩm để biểu thức sau mang giá trị âm

a.$\frac{x-2}{6-x}$

b.$\left(x-1\right)\left(x-2\right)\left(x-3\right)$