Question

6+ 9 + 12+........ + 63, find the sum
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Answer 1

Answer:

6+9+12+….......+63

i) It's clear that is is in A.P

ii)So, a=6. ;d=3. ;n=?

iii)now for n,

Tn= a+(n-1)d

63=6+(n-1)3

57/3+1=n

n= 20.

iv)now at last,

S=n/2(2a+(n-1)d

so,

S=20/2[2(6)+(20-1)3]

S=10[12+57]

S=690