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8)
![frac{20x}{2x-a}=7x-1;\
x=3;\
frac{20cdot3}{2cdot3-a}=7cdot3-1;\
frac{60}{6-a}=21-1;\
frac{60}{6-a}=20;\
60=20cdot(6-a);\
6-a=3;\
a=6-3=3;\
a=3;\](https://tex.z-dn.net/?f=frac%7B20x%7D%7B2x-a%7D%3D7x-1%3B%5C%0A+x%3D3%3B%5C+%0Afrac%7B20cdot3%7D%7B2cdot3-a%7D%3D7cdot3-1%3B%5C%0A+frac%7B60%7D%7B6-a%7D%3D21-1%3B%5C%0A+frac%7B60%7D%7B6-a%7D%3D20%3B%5C+%0A60%3D20cdot%286-a%29%3B%5C%0A+6-a%3D3%3B%5C%0A+a%3D6-3%3D3%3B%5C%0A+a%3D3%3B%5C "frac{20x}{2x-a}=7x-1;\
x=3;\
frac{20cdot3}{2cdot3-a}=7cdot3-1;\
frac{60}{6-a}=21-1;\
frac{60}{6-a}=20;\
60=20cdot(6-a);\
6-a=3;\
a=6-3=3;\
a=3;\")
![frac{20x}{2x-3}=7x-1;\
D(f): 2x-3neq0; 2xneq3; xneqfrac32;\
20x=(7x-1)cdot(2x-3);\
20x=14x^2-21x-2x+3=0;\
14x^2-43x+3=0;\
D=43^2-4cdot3cdot14=1681=(pm41)^2;\
x_1=frac{43-41}{28}=frac{2}{28}=frac{1}{14};\
x_2=frac{43+41}{28}=frac{84}{28}=frac{21}{7}=3;\
x=frac{1}{14}.](https://tex.z-dn.net/?f=frac%7B20x%7D%7B2x-3%7D%3D7x-1%3B%5C%0AD%28f%29%3A++2x-3neq0%3B++2xneq3%3B++xneqfrac32%3B%5C%0A20x%3D%287x-1%29cdot%282x-3%29%3B%5C%0A20x%3D14x%5E2-21x-2x%2B3%3D0%3B%5C%0A14x%5E2-43x%2B3%3D0%3B%5C%0AD%3D43%5E2-4cdot3cdot14%3D1681%3D%28pm41%29%5E2%3B%5C%0Ax_1%3Dfrac%7B43-41%7D%7B28%7D%3Dfrac%7B2%7D%7B28%7D%3Dfrac%7B1%7D%7B14%7D%3B%5C%0Ax_2%3Dfrac%7B43%2B41%7D%7B28%7D%3Dfrac%7B84%7D%7B28%7D%3Dfrac%7B21%7D%7B7%7D%3D3%3B%5C%0Ax%3Dfrac%7B1%7D%7B14%7D. "frac{20x}{2x-3}=7x-1;\
D(f): 2x-3neq0; 2xneq3; xneqfrac32;\
20x=(7x-1)cdot(2x-3);\
20x=14x^2-21x-2x+3=0;\
14x^2-43x+3=0;\
D=43^2-4cdot3cdot14=1681=(pm41)^2;\
x_1=frac{43-41}{28}=frac{2}{28}=frac{1}{14};\
x_2=frac{43+41}{28}=frac{84}{28}=frac{21}{7}=3;\
x=frac{1}{14}.")
3)
![V_c=15 kmcdot h;\
T=3 h 45 min\
L=27 km\
frac{L}{V_c-V_0}+frac{L}{V_c+V_0}=T;\
Lleft(frac{V_c+V_0+V_c-V_0}{(V_c^2-V_0^2)}right)=T;\
Lfrac{2V_c}{V_c^2-V_0^2}=T;\
V_c^2-V_0^2=frac{L}{T}cdot2V_c;\
V_0^2=V_ccdotleft(V_c-2frac{L}{T}right)=15(15-2frac{27}{3frac34})=15^2-30frac{}{}cdotfrac{27}{frac{15}{4}}=225-27cdot8=\
=225-216=9;\
V_0=3 km/h](https://tex.z-dn.net/?f=V_c%3D15+kmcdot+h%3B%5C%0AT%3D3+h+45+min%5C%0AL%3D27+km%5C%0Afrac%7BL%7D%7BV_c-V_0%7D%2Bfrac%7BL%7D%7BV_c%2BV_0%7D%3DT%3B%5C%0ALleft%28frac%7BV_c%2BV_0%2BV_c-V_0%7D%7B%28V_c%5E2-V_0%5E2%29%7Dright%29%3DT%3B%5C%0ALfrac%7B2V_c%7D%7BV_c%5E2-V_0%5E2%7D%3DT%3B%5C%0AV_c%5E2-V_0%5E2%3Dfrac%7BL%7D%7BT%7Dcdot2V_c%3B%5C%0AV_0%5E2%3DV_ccdotleft%28V_c-2frac%7BL%7D%7BT%7Dright%29%3D15%2815-2frac%7B27%7D%7B3frac34%7D%29%3D15%5E2-30frac%7B%7D%7B%7Dcdotfrac%7B27%7D%7Bfrac%7B15%7D%7B4%7D%7D%3D225-27cdot8%3D%5C%0A%3D225-216%3D9%3B%5C%0AV_0%3D3+km%2Fh "V_c=15 kmcdot h;\
T=3 h 45 min\
L=27 km\
frac{L}{V_c-V_0}+frac{L}{V_c+V_0}=T;\
Lleft(frac{V_c+V_0+V_c-V_0}{(V_c^2-V_0^2)}right)=T;\
Lfrac{2V_c}{V_c^2-V_0^2}=T;\
V_c^2-V_0^2=frac{L}{T}cdot2V_c;\
V_0^2=V_ccdotleft(V_c-2frac{L}{T}right)=15(15-2frac{27}{3frac34})=15^2-30frac{}{}cdotfrac{27}{frac{15}{4}}=225-27cdot8=\
=225-216=9;\
V_0=3 km/h")
скорость течения реки 3 км/час
3)
скорость течения реки 3 км/час
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