Page 257 - aljaber

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.a2 = a1r = 5 ×(−2) = −5 a1 = 5 r = −2 ()
2 2

.−5

.a :( )

a, a + 1, a + 2, …, a + 17

.1 a

S18 = 18a + (1 + 2 + 3 + + 17) = 18a + 17 × 18
2
= 18a + 9 × 17 = 9(2a + 17)

S18 9
…4 321
. 2a + 17

a=4 2a + 17 a

.S18 = 9(2 × 4 + 17) = 225

.a1r 3 = 2 a1 = 1 :( ) ( )
27 4

.r = 3 8 = 2 .r 3 = a1r 3 = 2 / 27 = 8

27 3 a1 1 / 4 27

x = 1×2 = 1 :( ) ()
43 6

y = 1×2 = 1
63 9

.x + y = 1 + 1 = 5
6 9 18
2, x, 6