Question

帮忙解题 要过程

Answer 1

1.
1)原式=3lg3-2lg7=3*0.4771-2*0.8451=-0.2589
2)原式=1/3(lg3+lg7)=1/3*(0.4771+0.8451)=0.4407
3)原式=lg7-5lg3=0.8451-5*0.4771=-1.5404
4)原式=lg7+2lg3+1=0.8451+2*0.4771+1=2.7993
5)原式=-3/2lg7=-3/2*0.8451=-1.2677
6)看不清
7)原式=2lg3+2-20lg7=2*0.4771+2-20*0.8451=-13.9478

2.
1)原式=loga(3)+2-loga(3)=2
2)原式=2lga/(5lga)-2lgb/(5lgb)=2/5-2/5=0

追问

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要过程

追答

1、令公差为d
S9=9a1+9*8/2*d
S4=4a1+4*3/2*d
9+9*8/2*d=4+4*3/2*d
d=-1/6
ak+a4=a1+(k-1)d+a1+3d=0
k=10

2、an=a1+(n-1)d
a1+6n=28 ①
Sn=na1+n(n-1)d/2
na1+3n(n-1)=28 ②
联立①②得：n=7,a1=-14

3、a1+a2+a3+a4+a5
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6
=1-1/6
=5/6

4、an=a1+(n-1)d
a1=13-2n ①
Sn=na1+n(n-1)d/2
na1+n(n-1)=35 ②
联立①②得：n=5,a1=3；n=7,a1=-1

5、令公差为d
a1+a1+d=20
a1+2d+a1+3d=120
解得：a1=-2.5,d=25
a5+a6=a1+4d+a1+5d=220