xiaoloi
幼苗
共回答了18个问题采纳率:83.3% 举报
①由(a+b+c)(a+b-c)=3ab,得到(a+b) 2 -c 2 =3ab,化简得:a 2 +b 2 -c 2 =ab,
则cosC=
a 2 + b 2 - c 2
2ab =
ab
2ab =
1
2 ,根据C∈(0,180°),得到∠C=60°,所以此选项错误;
②因为
S 10
10 =
10 a 1 +
10×9
2 d
10 =a 1 +
9
2 d,同理
S 100
100 =a 1 +
99
2 d,
S 110
110 =a 1 +
109
2 d,
则
S 100
100 -
S 10
10
100-10 =
( a 1 +
99
2 d)-( a 1 +
9
2 d)
90 =
d
2 =
S 110
110 -
S 100
100
110-100 =
( a 1 +
109
2 d)-( a 1 +
99
2 d)
10 =
d
2 ,
所以三点 (10,
S 10
10 ),(100,
S 100
100 ),(110,
S 110
110 ) 共线.此选项正确;
③根据等差数列的性质可知,S 10 ,S 20 -S 10 ,S 30 -S 20 成等差数列,
得到:2(S 20 -S 10 )=S 10 +(S 30 -S 20 ),将S 10 =30,S 20 =100,
代入得:2(100-30)=30+(S 30 -100),解得:S 30 =210.此选项正确;
④因为f(x)+f(1-x)=
1
2 x +
2 +
1
2 1-x +
2
=
1
2 x +
2 +
2 x
2 +
2 • 2 x =
2
2 ( 2 x +
2 ) +
2 x
2+
2 • 2 x
=
2 + 2 x
1 +
2 • 2 x =
2 + 2 x
2 (
2 + 2 x ) =
2
2 ,
则f(-8)+f(-7)+…+f(0)+…+f(8)+f(9)=
2
2 ×9=
9
2
2 .此选项正确.
所以,正确的结论序号有:②③④.
故答案为:②③④
1年前
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