Given that 2!17!1+3!16!1+4!15!1+5!14!1+6!13!1+7!12!1+8!11!1+9!10!1=1!18!N find the greatest integer that is less than 100N.
Solution — click to reveal
Multiplying both sides by 19! yields: 2!17!19!+3!16!19!+4!15!19!+5!14!19!+6!13!19!+7!12!19!+8!11!19!+9!10!19!=1!18!19!N.(219)+(319)+(419)+(519)+(619)+(719)+(819)+(919)=19N. Recall the Combinatorial Identity 219=∑n=019(n19). Since (n19)=(19−n19), it follows that ∑n=09(n19)=2219=218. Thus, 19N=218−(119)−(019)=218−19−1=(29)2−20=(512)2−20=262124. So, N=19262124=13796 and ⌊100N⌋=137.