[m]=\frac{12}{30}+\frac{5}{30}=\frac{12+5}{30}=\frac{17}{30}[/m]
[m]\frac{17}{30}:\frac{17}{36}=\frac{17}{30}\cdot \frac{36}{17}=\frac{36}{30}=\frac{12}{10}=1,2[/m]
21+21х +8х^2 =5х–9+6х^2
21+21х +8х^2 -5х+9-6х^2=0
2x^2+16x+30=0
x^2+8x+15=0
D=64-4*15=4
x_(1)=(-8-2)/2=[b]-5[/b]; x_(2)=(-8+2)/2=[b]-3[/b]