问题1.证明接下来的每个进展都是GP。另外,在每种情况下找到共同的比例:
(i)4,-2,1,-1 / 2,……。
(ii)-2 / 3,-6,-54,…………。
(iii)a,3a 2 /4,9a 3 /16,……。
(iv)1 / 2、1 / 3、2 / 9、4 / 27,……。
解决方案:
(i) Given a1 = 4, a2 = -2, a3 = 1
a2/a1 = -2/4 = -1/2
a3/a2 = -1/2
a4/a1 = (-1/2)/1 =-1/2
Now, a2/a1 = a3/a2 = a4/a3 = -1/2
Thus, a1, a2, a3, a4,… are in GP and common ratio(r) is -1/2.
(ii) Given a1 = -2/3, a2 = -6, a3 = -54
a2/a1 = -6/(-2/3) = 9
a3/a2 = -54/-6 = 9
Now, a2/a1 = a3/a2 = 9
Thus, a1, a2, a3,…… are in GP and common ratio(r) is 9.
(iii) Given a1= a, a2= 3a2/4, a3 = 9a3/16
a2/a1 = (3a2/4)/a = 3a/4
a3/a2 = (9a3/16)/(3a2/4) = 3a/4
Now, a2/a1 = a3/a2 = 9
Thus, a1, a2, a3,…… are in GP and common ratio(r) is 3a/4.
(iv) Given a1=1/2, a2=1/3, a3=2/9
a2/a1 = (1/3)/(1/2) = 2/3
a3/a2 = (2/9)/(1/3) = 2/3
Now, a2/a1 = a3/a2 = 2/3
Thus, a1, a2, a3,…… are in GP and common ratio(r) is 2/3.
问题2。证明由an = 2 /(3 n )定义的序列是nGP。
解决方案:
Given an = 2/(3n)
We put n=1,2,3,….., n∈N
The sequence is 2/3, 2/(32), 2/(33), ………..
a2/a1 = (2/(32))/(2/3)=1/3
a3/a2 = (2/(33))/(2/(32)) = 1/3
Now, a2/a1 = a3/a2 = 1/3
Thus, a1,a2,a3,…… are in GP and common ratio(r) is 1/3.
Hence, the given sequence is in GP.
问题3.查找:
(i)GP 1,4,16,64,……的第9个任期。
(ii)GP的第十个任期-3 / 4、1 / 2,-1 / 3,-2 / 9,…………。
(iii)GP 0.3、0.06、0.012,…………..的第八项。
(iv)GP 1 /(a 3 x 3 ),ax,a5x 5 ,…..的第十二项。
(v)GP√3,1/√3,1/3√3,………。的第n个项。
解决方案:
(i) Given a2/a1 = 4/1 = 4
a3/a2 = 16/4 = 4
Hence r = a3/a2 = a2/a1 = 4 and a1 = 1,
a9 = a1*r8
a9 = 1*(4)8
= (4)8
(ii) Given a2/a1 = (1/2)/(-3/4) = -2/3
a3/a2 = (-1/3)/(1/2) = -2/3
Hence, r = a3/a2 = a2/a1 = -2/3 and a1 = -3/4
a10 = a1*r9
a10 = (-3/4)*(-2/3)9
= (1/2)(2/3)8
(iii) Given a2/a1 = 0.06/0.3 = 0.2
a3/a2 = 0.012/0.06 = 0.2
Hence r = a3/a2 = a2/a1 = 0.2 and a1 = 0.3
a8 = a1*r7
a8 = 0.3*(0.2)7
(iv) Given a2/a1 = (ax)/(1/(a3x3)) = a4x4
a3/a2 = (a5x5)/(ax) = a4x4
Hence r = a3/a2 = a2/a1= a4x4 and a1 =1/(a3x3)
a12 = a1*r11
a8 = (1/(a3x3))*((a4x4)11)
= a41x41
(v) Given a2/a1 = (1/√3)/√3 = 1/3
a3/a2 = (1/3√3)/(1/√3) = 1/3
Hence r = a3/a2 = a2/a1= 1/3 and a1 = √3
an = a1*r(n-1)
= √3*(1/3)(n-1)
问题4.从GP 2 / 27、2 / 9、2 / 3,………,162的末尾找到第四个项。
解决方案:
Given a2/a1 = (2/9)/(2/27) = 3
a3/a2 = (2/3)/(2/9) = 3
Hence r = a3/a2 = a2/a1= 3 and a1 = 2/27
After reversing the GP it becomes 162, ……….,, 2/3, 2/9, 2/27.
a1’=162 and r’= 1/r =1/3
a4’= a1′ * r’3
= 162 * (1/3)3
= 6.
问题5.进度的哪个项0.004,0.02,0.1,…是12.5?
解决方案:
Given a2/a1= 0.02/0.004= 5
a3/a2 = 0.1/0.02 = 5
Hence, r=a3/a2 = a2/a1= 5 and a1=0.004
nth term of sequence is given by
an = a1*(r)n-1
an=(0.004)*(5)n-1
Let the nth term be 12.5.
Hence, (0.004)*(5)n-1=12.5
(5)n-1= 12.5/0.004
(5)n-1= 3125
(5)n-1= (5)5
Hence, n-1=5
n=5+1= 6
问题6. GP的哪个任期:
(i)√2,1 /√2,1 /2√2,1 /4√2,…………..是1 /(512√2)吗?
(ii)2、2√2、4,………………..是128吗?
(iii)√3,3,3√3,……………………。是729吗?
(iv)1 / 3、1 / 9、1 / 27,…………是1/19683吗?
解决方案:
(i) In given sequence a1=√2, a2= 1/√2
Common ratio(r) = a2/a1 = (1/√2)/√2 =1/2 and a1 = √2
Let the nth term be 1/(512√2)
a1*rn-1 = 1/(512√2)
√2*(1/2)n-1 = 1/(512√2)
(1/2)n-1 = 1/1024
(1/2)n-1 = (1/2)10
Hence, n-1= 10
n=10+1 =11.
(ii) In given sequence a1=2 ,a2= 2√2
Common ratio(r)=a2/a1= 2√2/2 = √2 and a1=2
Let the nth term be 1/(512√2)
a1*rn-1 = 128
2*(√2)n-1= 128
(√2)n-1=64
(√2)n-1= (√2)12
Hence n-1= 12
n= 12+1 =13.
(iii) In given sequence a1= √3, a2= 3
Common ratio(r)=a2/a1 = 3/√3 = √3 and a1 = √3
Let the nth term be 729
a1*rn-1 = 729
√3*(√3)n-1= 729
(√3)n = 729
(√3)n = (√3)11
Hence n-1= 11
n= 11+1 =12.
(iv) In given sequence a1= 1/3, a2= 1/9
Common ratio(r)=a2/a1 = 1/3 and a1 = 1/3
Let the nth term be 1/19683
a1*rn-1 = 1/19683
1/3*(1/3)n-1 = 1/19683
(1/3)n = 1/19683
(1/3)n = (1/3)8
Hence n = 8.
问题7.哪个进展18,-12、8,…………..是512/729?
解决方案:
In given sequence a1=18 ,a2 =-12
Common ratio(r)= -12/18 = -2/3 and a1=18
Let the nth term be 512/729
a1*rn-1 = 512/729
(18)*(-2/3)n-1 = 512/729
(-2/3)n-1 = 256/6561
(-2/3)n-1 = (-2/3)8
Hence n-1 = 8
n = 8+1 = 9
问题8.从GP 1/2 1 / 6、1 / 18、1 / 54,…………..1 / 4373的末尾找到第四个项?
解决方案:
In given sequence a1 = 1/2, a2 = 1/6
Common ratio(r)= (1/6)/(1/2) =1/3 and a1 = 1/2
After reversing the GP it becomes 1/4374,……..1/54, 1/18, 1/6, 1/2
Common ratio(r’)= 3 and a1 = 1/4374
4th term from end of original GP becomes 4th from beginning of reversed GP
Hence required term is given by
(a1′)*(r’4-1) = (a1′)*(r’)3
= (1/4374)*(3)3
= (27/4374)
= 1/162
问题9. GP的第四学期是27,第七学期是729,找到GP吗?
解决方案:
Let a be first term of required GP and r be common ratio.
Given, a4= 27 and a7 = 729
(a1*r6) / (a1*r3) = 729/27
r3 = 729/27 = 27
r3 = 33
r=3
Now, put r=3 in a1*r3=27
a1*(3)3 = 27
a1 = 1
Thus, the given GP is 1,3, 9, 27,……….
问题10. GP的第7个学期是第4个学期的8倍,第5个学期是48个学期。
解决方案:
Let a be first term of required GP and r be common ratio.
Given, a7 = 8*a4 and a5 = 48
a7/a4 = 8
(a1*r6)/(a1*r3) = 8
r3 = 8
r=2
Now put r = 2 in a5 = 48
a1*r4 = 48
a1* (2)4 = 48
a1* 16 = 48
a1 = 3
Thus, the given GP is 3, 6, 12, 24,…………..