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:( ) ()
90 + 91 + 92 + 93 + 94 + 95 + 96 + 97 + 98 + 99
= 90 ×10 + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9
= 90 ×10 + (1 + 9) + (2 + 8) + (3 + 7) + (4 + 6) + 5
= 900 + 10 + 10 + 10 + 10 + 5
= 945
:
S = 90 + 91 + 92 + 93 + 94 + 95 + 96 + 97 + 98 + 99
S = 99 + 98 + 97 + 96 + 95 + 94 + 93 + 92 + 91 + 90
2S = 189 + 189 + 189 + + 189 = 189 × 10 = 1890 ()
10
.S = 1890 = 945
2
:( )
10
. 107 = 103 = 1000 = 200
5 ×104 5 5
:( ) ()
. 3a = −6 a2 = 4 24 = −12 4a = −8 −3a = 6
a
−3a
.
:( )
( )
.1
3
