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प्रश्न
For an A.P., find S7 if a = 5 and d = 4.
उत्तर
a = 5, d = 4
`S_n = n/2[2a +(n -1)d]`
`S_7 = 7/2[2(5) + (7 - 1)(4)]`
`= 7/2 [10 + 24]`
`= 7/2 xx 34`
S7 = 119
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