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Є таке правило: добуток крайніх членів пропорції дорівнює добутку середніх її членів.
1)
![36 \div (x + 3) = 54 \div 3 \\ 54(x + 3) = 36 \times 3 \\ 54x + 162 = 108 \\ 54x = 108 - 162 \\ 54x = - 54 \\ x = - 1. 36 \div (x + 3) = 54 \div 3 \\ 54(x + 3) = 36 \times 3 \\ 54x + 162 = 108 \\ 54x = 108 - 162 \\ 54x = - 54 \\ x = - 1.](https://tex.z-dn.net/?f=36+%5Cdiv+%28x+%2B+3%29+%3D+54+%5Cdiv+3+%5C%5C+54%28x+%2B+3%29+%3D+36+%5Ctimes+3+%5C%5C+54x+%2B+162+%3D+108+%5C%5C+54x+%3D+108+-+162+%5C%5C+54x+%3D++-+54+%5C%5C+x+%3D++-+1.)
2)
![2 \div 3 = (5x - 7) \div 12 \\ 3(5x - 7) = 12 \times 2 \\ 15x - 21 = 24 \\ 15x = 24 + 21 \\ 15x = 45 \\ x = 3 2 \div 3 = (5x - 7) \div 12 \\ 3(5x - 7) = 12 \times 2 \\ 15x - 21 = 24 \\ 15x = 24 + 21 \\ 15x = 45 \\ x = 3](https://tex.z-dn.net/?f=2+%5Cdiv+3+%3D+%285x+-+7%29+%5Cdiv+12+%5C%5C+3%285x+-+7%29+%3D+12+%5Ctimes+2+%5C%5C+15x+-+21+%3D+24+%5C%5C+15x+%3D+24+%2B+21+%5C%5C++15x+%3D+45+%5C%5C+x+%3D+3)
3)
![0.45 \div (x + 0.8) = 6 \div 20 \\ 6(x + 0.8) = 20 \times 0.45 \\ 6x + 4.8 = 20 \times \frac{45}{100} \\ 6x + 4.8 = \frac{45}{5} \\ 6x = 9 - 4.8 \\ 6x = 4.2 \\ x = 0.7 0.45 \div (x + 0.8) = 6 \div 20 \\ 6(x + 0.8) = 20 \times 0.45 \\ 6x + 4.8 = 20 \times \frac{45}{100} \\ 6x + 4.8 = \frac{45}{5} \\ 6x = 9 - 4.8 \\ 6x = 4.2 \\ x = 0.7](https://tex.z-dn.net/?f=0.45+%5Cdiv+%28x+%2B+0.8%29+%3D+6+%5Cdiv+20+%5C%5C+6%28x+%2B+0.8%29+%3D+20+%5Ctimes+0.45+%5C%5C+6x+%2B+4.8+%3D+20+%5Ctimes++%5Cfrac%7B45%7D%7B100%7D++%5C%5C+6x+%2B+4.8+%3D++%5Cfrac%7B45%7D%7B5%7D++%5C%5C+6x+%3D+9+-+4.8+%5C%5C+6x+%3D+4.2+%5C%5C+x+%3D+0.7)
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