Giải

                                        Chu vi của mảnh vườn là:

                                                            (5,5 + 3,75).2 = 18,5 (m)

                                        Số khóm hoa cần trồng là:

                                                               18,5 : \(\dfrac{1}{4}\) = 18,5. 4 = 74 (khóm hoa)

                                                                                            Đáp số: 74 khóm hoa

a) \(\dfrac{1}{5}\)+\(\dfrac{4}{5}\): x = 0,75

   => \(\dfrac{4}{5}\) : x = \(\dfrac{1}{4}\) - \(\dfrac{1}{5}\)

   =>  \(\dfrac{4}{5}\) : x = \(\dfrac{5}{20}\) - \(\dfrac{4}{20}\)

   =>  \(\dfrac{4}{5}\) : x = \(\dfrac{1}{20}\)

        =>   x = \(\dfrac{4}{5}\) : \(\dfrac{1}{20}\)

          => x = \(\dfrac{4}{5}\) x 20 = \(\dfrac{80}{5}\) = 16

                    Vậy x = 16

b) x + \(\dfrac{1}{2}\) = 1 - x

=>  x + x = 1 - \(\dfrac{1}{2}\)

   =>  2.x = \(\dfrac{1}{2}\)

      =>  x = \(\dfrac{1}{2}\) : 2

       => x = \(\dfrac{1}{2}\).\(\dfrac{1}{2}\) = \(\dfrac{1}{4}\)

                Vậy x = \(\dfrac{1}{4}\)

a) \(\dfrac{2}{3}\).(\(\dfrac{5}{4}\)-\(\dfrac{3}{4}\))

= \(\dfrac{2}{3}\).\(\dfrac{2}{4}\)= \(\dfrac{2}{3}\).\(\dfrac{1}{2}\)

= \(\dfrac{1}{3}\)

b) 2. (\(\dfrac{-3}{2}\))² -\(\dfrac{7}{12}\)

= 2. \(\dfrac{9}{4}\) - \(\dfrac{7}{12}\)

= \(\dfrac{18}{4}\) - \(\dfrac{7}{12}\)

= \(\dfrac{54}{12}\) - \(\dfrac{7}{12}\)

= \(\dfrac{47}{12}\)

c) \(\dfrac{-3}{4}\).5\(\dfrac{3}{13}\) - 0,75.\(\dfrac{36}{13}\)

= \(\dfrac{-3}{4}\).\(\dfrac{68}{13}\) - \(\dfrac{3}{4}\).\(\dfrac{36}{13}\)

= \(\dfrac{3}{4}\). (\(\dfrac{-68}{13}\) - \(\dfrac{36}{13}\))

= \(\dfrac{3}{4}\) . \(\dfrac{-104}{13}\)

= \(\dfrac{3}{4}\) . -8

= \(\dfrac{-24}{4}\) = -6