Source: Mu Alpha Theta
Check their website over here: https://mualphatheta.org/
Evaluate:
(150)+3(151)+....+31(1515)
Your answer should be in the form of 2^n.
Hint:https://web2.0calc.com/questions/double-counting
listfor(n, 0, 15, ((2*n + 1)*(15 nCr n))==(1, 45, 525, 3185, 12285, 33033, 65065, 96525, 109395, 95095, 63063, 31395, 11375, 2835, 435, 31)>>>Sum==524,288
Interesting Problem!!!!!!
Let S=15C0+3(15C1)+5(15C2)+7(15C3)+...+31(15C15)
Then, S=(15C15)+3(15C14)+...+31(15C0) (Using the property that: nCk=nC(n−k) ).
Adding the expressions above, to get:
2S=(15C0)(31+1)+(15C1)(3+29)+...+(15C15)(31+1)
Hence, 2S=32(15C0+15C1+15C2+...+15C15)=32∗215=25∗215=220
Therefore, S=219