Question
S=1×6+2×7+3×8+...............+15×20, what is the value of S?
Answer: Option B
:
B
Tn=n(n+5)=n2+5n
Sn=∑Tn=∑n2+5∑n
=n(n+1)(2n+1)6+5×n(n+1)2; Put n = 15
Sn= 1840
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:
B
Tn=n(n+5)=n2+5n
Sn=∑Tn=∑n2+5∑n
=n(n+1)(2n+1)6+5×n(n+1)2; Put n = 15
Sn= 1840
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