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P. 259
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( ). ()
1 S6 = 1 2 26 − 1 = 2 × 63 = 7
18 × 2−1 18
18
.a1, a1r, a1r 2, … :( )
. 0, d, 2d, …
a1 + 0 = 1 ⇒ a1 = 1 cn = an + bn
( ) a1r + d = 1 ⇒ r + d = 1
( ) a1r2 + 2d = 2 ⇒ r2 + 2d = 2
( ) −2 ( )
.r(r − 2) = 0 .r 2 − 2r = 0
.d = −1 .(r ≠ 0 ) r = 2
{an }
( )a1 r10 − 1 = 210 − 1 = 1023
r −1 2 −1
{bn }
10 0 + 9 ×(−1) = −45
2
{cn }
.1023 + (−45) = 978
.d = 1 a1 = k2 + 1 :( ) ()