Question

en una progresion arimetica a20=-33 y a12=-28, hallat a1 y d

Answer 1

Fernanda,
Vamos paso a paso

Usamos la fórmula general
         an = a1 + (n -1)*d

En el caso en estudio
                         a12 = - 28
                         a20 = - 33
                             n = 9 (de 12 a 20)
                             d = ??
                                                             - 33 = - 28 + (9 - 1)*d
                                                     - 33 + 28 = 8*d
                                                               - 5 = 8d
                                                                                d = - 5/8
                      - 28 = a1 + (12 - 1)*(-5/8)
                             = a1 + 11*(- 5/8)
           - 28 + 55/8 = a1
       (- 224 + 55)/8 = a1
                                                        a1 = 169/8