P=\(\dfrac{2}{3}\)-(-\(\dfrac{1}{4}\))+\(\dfrac{3}{5}\)-\(\dfrac{7}{45}\)-(-\(\dfrac{5}{9}\))+\(\dfrac{1}{12}\)+\(\dfrac{1}{35}\)

=\(\dfrac{2}{3}\)+\(\dfrac{1}{4}\)+\(\dfrac{3}{5}\)-\(\dfrac{7}{45}\)+\(\dfrac{5}{9}\)+\(\dfrac{1}{12}\)+\(\dfrac{1}{35}\)

=(\(\dfrac{2}{3}\)+\(\dfrac{1}{4}\)+\(\dfrac{1}{12}\))+(\(\dfrac{3}{5}\)-\(\dfrac{7}{45}\)+\(\dfrac{5}{9}\))+\(\dfrac{1}{35}\)

=(\(\dfrac{8}{12}\)+\(\dfrac{3}{12}\)+\(\dfrac{1}{12}\))+(\(\dfrac{27}{45}\)-\(\dfrac{7}{45}\)+\(\dfrac{25}{45}\))+\(\dfrac{1}{35}\)

=1+1+\(\dfrac{1}{35}\) =\(2\dfrac{1}{35}\)

a) A = (\(\dfrac{1}{3}\) - \(\dfrac{8}{15}\) - \(\dfrac{1}{7}\)) + (\(\dfrac{2}{3}\) + \(\dfrac{-7}{15}\) + \(1\dfrac{1}{7}\))

       = \(\dfrac{1}{3}\) - \(\dfrac{8}{15}\) - \(\dfrac{1}{7}\) + \(\dfrac{2}{3}\) - \(\dfrac{7}{15}\) + \(\dfrac{8}{7}\)

       = (\(\dfrac{1}{3}\) - \(\dfrac{8}{15}\) + \(\dfrac{2}{3}\) - \(\dfrac{7}{15}\)) + ( - \(\dfrac{1}{7}\) + \(\dfrac{8}{7}\))

       = (\(\dfrac{5}{15}\) - \(\dfrac{8}{15}\) + \(\dfrac{10}{15}\) - \(\dfrac{7}{15}\)) + 1

       = 0 + 1 = 1

b) B= 0,25 + \(\dfrac{3}{5}\) - (\(\dfrac{1}{8}\) - \(\dfrac{2}{5}\) + \(1\dfrac{1}{4}\))

      = \(\dfrac{1}{4}\) + \(\dfrac{3}{5}\) - \(\dfrac{1}{8}\) + \(\dfrac{2}{5}\) - \(\dfrac{5}{4}\)

      = \(\dfrac{10}{40}\) + \(\dfrac{24}{40}\) - \(\dfrac{5}{40}\) + \(\dfrac{16}{40}\) - \(\dfrac{50}{40}\)

      = \(\dfrac{-5}{40}\) = \(\dfrac{-1}{8}\)

5,6 cm

Giả thiết: Hai đường thẳng phân biệt cùng vuông góc với một đường thẳng thứ ba

Kết luận: Chúng song song với nhau

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