a. p = 21,18, 24 ... sn = 78 n= ?
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Heya☺
sn = n/2 ( 2a + (n-1)d )
sn = n/2 ( 2× 21 + (n-1) × -3 ).
sn = n/2 ( 42 -3n +3)
78 ×2= n ( 45 -3n)
156 = 45n - 3n^2
n^2 - 15n + 52 =0
n^2
Pls check ur question once , I feel there is a mistake in A.P
Because the difference between subsequent terms should be same .
Check it once!
sn = n/2 ( 2a + (n-1)d )
sn = n/2 ( 2× 21 + (n-1) × -3 ).
sn = n/2 ( 42 -3n +3)
78 ×2= n ( 45 -3n)
156 = 45n - 3n^2
n^2 - 15n + 52 =0
n^2
Pls check ur question once , I feel there is a mistake in A.P
Because the difference between subsequent terms should be same .
Check it once!
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0
plz wright the correct question
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