Answers

  • Brainly User
2015-11-16T07:09:15+08:00
Arithmetic sequence:  39, 42, 45, ...72

a _{n} = a_{1} +(n-1)(d)

a _{n} = last term ⇒  72
 a_{1} = first term ⇒  39
d = common difference between any of the two consecutive sequence  ⇒  3
n = number of terms  ⇒  number of tests the student takes in Math

72 = 39 + (n - 1) (d)
72 = 39 + (-3) + 3n
72 = 36 + 3n
3n = 72 - 36
3n/3 = 36/3
n = 12

The student takes 12 tests in Math.

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