Point: 50
2×\sum^{n}_{k=0}k¹
\fbox{What means by this?}
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No one know it? Okay-
\sum^{n}_{k=0}k¹ ×2
=2×(1+2+3+…+n)
=2×1+2×2+2×3+…+2n
=2+4+6+…+2n –> সব ধনাত্মক জোড় পূর্ণসংখ্যার যোগফল
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