"Fibonacci Sequence II"
Given the first few numbers of the Fibonacci sequence:
1 1 2 3 5 8 13 21 ...
What is the first Fibonacci number to contain 15 digits?
Give the actual 15 digit number for the answer.
Compared to the previous couple this one was a walk in the park, learn how the sequence works (trivial) create a loop that exits once the current number is 15 digits long and voila!
Awnser:
1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181 6765 10946 17711 28657 46368 75025 121393 196418 317811 514229 832040 1346269 2178309 3524578 5702887 9227465 14930352 24157817 39088169 63245986 102334155 165580141 267914296 433494437 701408733 1134903170 1836311903 2971215073 4807526976 7778742049 12586269025 20365011074 32951280099 53316291173 86267571272 139583862445 225851433717 365435296162 591286729879 956722026041 1548008755920 2504730781961 4052739537881 6557470319842 10610209857723 17167680177565 27777890035288 44945570212853 72723460248141 117669030460994
= 117669030460994
= 117669030460994
Source:
function solution(){ $prevno = 1; $currno = 1; echo $prevno." "; while( strlen($currno) < 15 ){ $placeholder = bcadd($prevno, $currno); $prevno = $currno; $currno = $placeholder; echo $currno." "; } echo "
= ".$currno.""; }