Problem Statement
(a_n)_{n \equal{} 1}^\infty is defined on real numbers with a_n \not \equal{} 0, a_na_{n \plus{} 3} \equal{} a_{n \plus{} 2}a_{n \plus{} 5} and a_1a_2 \plus{} a_3a_4 \plus{} a_5a_6 \equal{} 6. So a_1a_2 \plus{} a_3a_4 \plus{} \cdots \plus{}a_{41}a_{42} \equal{} ?<spanclass=′latex−bold′>(A)</span> 21<spanclass=′latex−bold′>(B)</span> 42<spanclass=′latex−bold′>(C)</span> 63<spanclass=′latex−bold′>(D)</span> 882<spanclass=′latex−bold′>(E)</span> None