Прикрепил
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1)6m+12 (формула)
6*18+12=120 марок
6*36+12=228 марок
<span>3/4 = 9 / x+8
3(х+8)=4×9
3х+24=36
3х=36-24
3х=12
х=4
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1) 15 3/8 : (2 3/4х + 5 5/6) - 1 1/2 = 3/4
15 3/8 : (2 3/4х + 5 5/6) = 2 1/4
2 3/4х + 5 5/6 = 15 3/8 : 2 1/4
2 3/4х + 5 5/6 = 123/8 * 4/9
2 3/4х + 5 5/6 = 6 5/6
2 3/4х = 1
11/4х = 1
х = 4/11 => доказано
3 1/3 - (4 1/5х + х) : 5 4/7 = 8/15
3 1/3 - 5 1/5х = 8/15 * 39/7
3 1/3 - 5 1/5х = 8/5 * 13/7
26/5х = 3 1/3 - 2 34/35
26/5х = 3 35/105 - 2 102/105
26/5х = 350/105 - 312/105
26/5х = 38/105
х = 38/105 * 5/26
х = 19/21 * 1/13
х = 19/273
Может, ошибка в условии, но х не = 3.