Найди все формулы с ошибками

• \(\frac{a}{b} = \frac{a:n}{b:n}\)
• \(\frac{a}{b} : \frac{c}{d} = \frac{a ⋅ с}{b ⋅ d}\)
• \(\frac{a}{b} ⋅ с = \frac{a ⋅ с}{b}\)
• \(\frac{a}{b} + \frac{c}{b} = \frac{a+b}{c}\)