Question

find the sum of the series 51+50+49+......+21

Answer 1

Step-by-step explanation:

51 + 50 + 49 +..... + 21

a = 51, d = -1, an = 21

∴ a + (n-1) d = an

51 + (n-1)(-1) = 21

(n-1)(-1) = 21 - 51

n-1 = 30

∴ n = 31

∴ Sum of the series = 31/2(51+21)

= 31/2 × 72

= 1116

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Answer 2

Answer:

an=a+(n-1)d. (d=50-51=-1)

21=51+(n-1)(-1)

21-51=-n+1

-31=-n+1

n=1+31

n=32

Sum of AP =n/2{a+an}

=32/2(51+21)

=16(72)

=1152