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tmua.me Medium Set F Paper 2 · Q9

Paper 2Sequences & series

A sequence is defined by

u1=3,un+1=un+(−1)n+1nfor n≥1.u_1=3,\qquad u_{n+1}=u_n+(-1)^{n+1}n\quad\text{for }n\geq1.

What is the sum of all the positive terms among u1,u2,…,u20u_1,u_2,\ldots,u_{20}?

A7070
B8585
C8888
D9191
E112112

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