Write the explicit formula for the geometric sequence.

a1 = -5 a2 = 20 a3 = -80
A. an = -5 • (-4)n

B. an = -5(-4)n-1

C. an = -4(-5)n-1

D. an = -5 • (4)n

Question

Write the explicit formula for the geometric sequence.

a1 = -5  a2 = 20  a3 = -80
  A. an = -5 • (-4)n
 	
  B. an = -5(-4)n-1
 	
  C. an = -4(-5)n-1
 	
  D. an = -5 • (4)n

jmark · Answer

In geometric sequence, the formula to find the nth term is given below a_n = ar^(n-1), where a = first term, r = common difference, n = total number of term. Note: common difference can be found by dividing second term by the first term. Since a_1 = -5, a_2 = 20 and a_3 = -80 We are given with three terms so n = 3 r = a_2/a_1 = 20/-5 = -4 a_n = -5(-4)^2 CBSE Class 6 Math Question Paper - http://education-help-online.blogspot.in/2014/07/cbse-class-6-math-question-paper.html