<span>Предлагаю несколько вариантов на выбор.
а) e-f-a-b-c = -(f-e) - (a+b+c) = -(f-e+a+b+c) = -(f-e+a) - (b+c)
б) e+f-a-b+c = -(a-e-f) - (b-c) = -(a-e-f+b-c) = -(-e-f) - (a+b-c)
</span>в) a+b+c+e+f = -(-a-b) - (-c-e-f) = -(-a-b-c-e-f) = -(-a-b-c) - (-e-f)
г) a-b+c+e-f = -(b-a) - (f-c-e) = -(b-a-c-e+f) = -(b-a-c) - (f-e)
7/10 от 40 40*7/10=40:10*7=4*7=28
3/5 от 50 50*3/5=50:5*3=10*3=30
0,4 : 1,7 = (0,4 * 10) : (1,7 * 10) = 4 : 17
Или так:
![\displaystyle \tt \frac{4}{10}:1\frac{7}{10}=\frac{4}{10}:\frac{17}{10}=\frac{4}{10}\cdot\frac{10}{17}=\frac{4}{17}=4:17](https://tex.z-dn.net/?f=%5Cdisplaystyle%20%5Ctt%20%5Cfrac%7B4%7D%7B10%7D%3A1%5Cfrac%7B7%7D%7B10%7D%3D%5Cfrac%7B4%7D%7B10%7D%3A%5Cfrac%7B17%7D%7B10%7D%3D%5Cfrac%7B4%7D%7B10%7D%5Ccdot%5Cfrac%7B10%7D%7B17%7D%3D%5Cfrac%7B4%7D%7B17%7D%3D4%3A17)