Common difference of an A.P is -3and the 21st terms is -50 then find the sum of first 21 terms
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Answer: -420
Step-by-step explanation:
Common difference, d= -3
Let 'a' be the first term.
21st term = a + (21-1)d = a -60
given 21st term = -50
a - 60 = -50 => a = 10
Sum to n terms of an A.P. = (n/2)(2a+(n-1)d)
Sum to first 21 terms = (21/2)(20+(20×-3)) = -420
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