Question

For which of the following geometric series can the infinite sum be determined? a1 = –0.25, r = –3 a1 = 0.75, r = 4 a1 = 3, r = 2 a1 = 5, r = –0.5

Answer 1

So, the infinite sum can be determined in the geometric series a₁ = 5, r = -0.5.

For a geometric series the infinite sum is given by s = a/(1 - r) where a = first term, r = common ratio and |r| < 1.

For each of the geometic series given below, we need to determine if |r| < 1.

For a₁ = -0.25, r = - 3, |r| = |-3| = 3 > 1

For a₁ = 0.75, r = 4, |r| = |4| = 4 > 1

For a₁ = 3, r = 2, |r| = |2| = 2 > 1

For a₁ = 5, r = -0.5, |r| = |-0.5| = 0.5 < 1

Since we require |r| < 1 to find the infinite sum and |r| in the last geometric series |r| = 0.5 < 1, the infinite sum can be found in the geometric series a₁ = 5, r = -0.5..

So, the infinite sum can be determined in the geometric series a₁ = 5, r = -0.5.

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