Four consecutive terms of an arithmetic sequence are z, 2.x, 2y, and r-y+12.
What is the ninetieth term of that sequence?
Answers
Answer:
2y = 4x-8, or
y = 2x-4 …(1)
2x, 2y and 36 form a GP, or
36/2y or 18/y = 2y/2x or y/x
y^2 = 18x …(2)
Equate (1) and (2)
(2x-4)^2 =18x.
4x^2–16x+16 = 18, or
4x^2–34x+16 = 0
2x^2–17x+8 = 0
(2x-1)(x-8) = 0
x= 1/2 or 8
y = -3 or 12
Check: 8, 2x, 2y become 8, 1, -6. Correct as an AP.
2x, 2y, 36 become 1, -6, 36. Correct as an GP.
8, 2x, 2y become 8, 16, 24. Correct as an AP.
2x, 2y, 36 become 16, 24, 36. Correct as an GP.
x= 1/2 and y = -3, or
x = 8 and y = 12.
This is easy, I remember this one from Day School when I was pounding the Square Peg into the Round Hole with a Rubber Mallot. Because ANY sequence MUST comprehend as part of Why Newton debated: For theory of everything up is also a theory of everything down, to come to his latter publications, Atwhich: Meeting a criterium of that.