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Question

The sum 1+13+231+2+13+23+331+2+3++13+23+33++1531+2+3++1512(1+2+3++15) is equal to :

A
620
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B
660
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C
1240
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D
1860
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Solution

The correct option is A 620
Sum =15n=113+23++n31+2++n1215×162
=15n=1(n(n+1)2)2n(n+1)260
=1215n=1n2+1215n=1n60
=12×15×16×316+12×15×16260
=620

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