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Note: 31 তম পদ= 10+(31-1)(-2) = 10+(-60) = -50

Note: `s = n/2{2a+(n-1)d} = 31/2{2 xx 10+(31-1)(-2) }= -620`

Note: 2a+19d = 56 2a+11d = 24 _____________ (-) করে , 8d = 32 `:.` d = 4 2a+11d = 24 বা, `2a+11 xx 4 = 24` বা , 2a = 24-44 = -20 বা, a = -10

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Note: `s_6 =6/2 {2 xx (-10)+(6-1)4} = 3(-20+20) = 0.`

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Note: 20- তম পদ = a+(20-1) = a+19d =24+19(-2) = 24-38 = -14

Note: এখানে,` a = 24, d = -2, n= 16` `:. s = n/2{2a + (n-1)d} = 16/2 { 2 xx 24 + (16-1).(-2)}` `= 8 (48-30) = 8 xx 18 = 144`