Question

How many terms of the series 17 , 15 , 13.... amount to 72? ​

Answer 1

Answer:

Step-by-step explanation:

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Answer 2

Answer:

a = 17

d = -2

Total sum = 72

Total sum = n/2 ( 2a + (n - 1)d

72 = n/2 ( 2×17 + (n - 1)(-2)

72 = n/2(34 -2n + 2)

72 = n/2 (36 - 2n)

72 = 36n/2 - n^2

72 = 18n - n^2

n^2 - 18n + 72 = 0

n^2 -6n - 12n + 72 = 0

n(n- 6) - 12(n - 6) = 0

(n - 6)(n - 12) = 0

Here,

n = 6 or 12

Now,

If n = 6

Total sum = 6/2 ( 2× 17 +( 6 - 1)(-2)

3(34 - 10)

3 × 24

72

Hence , n = 6 ans...