问题23.如果AP的第5和第12个学期分别是30和65,那么前20个学期的总和是多少?
解决方案:
According to question we have
a5 = 30
Using the formula
⇒ a + (5 – 1)d = 30
⇒ a + 4d = 30 ……………. (i)
Also, an = 65
Using the formula
⇒ a + (12 – 1)d = 65
⇒ a + 11d = 65 ……………(ii)
On solving eq(i) and (ii), we get
7d = 35
⇒ d = 5
On putting the value of d in (i), we get :
a + 4 x 5 = 30
⇒ a = 10
Now we find the sum of first 20 terms
S20 = 20/2 [2 x 10 + (20 – 1) x 5]
⇒ S20 = 10 [20 + 95]
⇒ S20 = 1150
问题24.找到第k个项为5k + 1的AP的n个项之和。
解决方案:
According to question we have
ak = 5k + 1
For k = 1, a1 = 5 x 1 + 1 = 6
For k = 2, a2 = 5 x 2 + 1 = 11
For k = n, an = 5n + 1
Now we find the sum of n terms
Sn = n/2 [a + an]
⇒ Sn = n/2[6 + 5n + 1] = n/2 (5n + 7)
问题25.找到所有两位数字的总和,当它们除以4时,得出1的余数。
解决方案:
According to question we have
A.P = 13, 17….97
From the A.P we have
a = 13, d = 4, an = 97
Now using the formula
⇒ an = a (n – 1)d
⇒ 97 = 13 + (n – 1)4
⇒ 84 = 4n – 4
⇒ 88 = 4n
⇒ 22 = n …………… (1)
Now we find the sum of all two-digit numbers using the given formula
Sn = n/2[2a + (n – 1)d]
S22 = 22/2 [2 x 13 + (22 – 1) x 4] ( From eq(1))
⇒ S22 = 11[26 + 84]
⇒ S22 = 11[110] = 1210
问题26.如果AP 25、22、19,…的一定数量的术语的总和是116。找到最后一个术语。
解决方案:
According to question
A.P. = 25, 22, 19
From the A.P we have
a = 25, d = 22 – 25 = -3
Sn = 116
Now using the formula
⇒ n/2 [2a + (n – 1)d] = 116
⇒ n [2 x 25 + (n – 1)(-3)] = 232
⇒ 50n -3n2 + 3n = 232
⇒ 3n2 – 53n + 232 = 0
⇒ 3n2 – 29n – 24n + 232 = 0
⇒ n(3n – 29) – 8(3n – 29) = 0
⇒(3n – 29)(n – 8) = 0
⇒ n = 29/ 3 or 8
Since n cannot be a fraction, n = 8.
Now we find the last term using the following formula
an = a + (n – 1)d
⇒ a8 = 25 + (8 – 1)(-3)
⇒ a8 = 4
问题27.找出1到2001之间的奇数整数之和。
解决方案:
According to question
A.P = 1, 3, 5 … 2001
From the A.P we have
a = 1 and d = 2
an = 2001
Now using the formula
⇒ 1 + (n – 1)2 = 2001
⇒ 2n – 2 = 2000
⇒ 2n = 2002
⇒ n = 1001
Now we find the sum of odd integers from 1 to 2001
Also, S1001= 1001/2 [2 x 1 + (1001 – 1)2]
⇒ S1001 = 1001/2 [2 + 2000]
⇒ S1001 = 1001 x 1001 = 1002001
问题28.要加上-25,需要多少个AP -6,-11 / 2,-5,…?
解决方案:
According to question
A.P = -6, -11/2, -5, …
From the A.P we have
a = – 6 and d =-11/2 – (-6) = 1/2
Sn = -25
Now using the formula
⇒ -25 = n/2 [2 x (-6) + (n -1)(1/2)]
⇒ -25 = n/2 [ -12 + n/2 – 1/2]
⇒ -50 = n [ n/2 – 25/2]
⇒ -100 = n [ n – 25]
⇒ n2 – 25n + 100 = 0
⇒ (n – 20)(n – 5) = 0
⇒ n = 20 or n = 5
问题29.在AP中,第一项为2,而前五项之和为后五项的四分之一。表明第20个学期是-112。
解决方案:
According to question we have
a = 2, S5 = 1/4(S10 – S5)
S5 = 5/2 [2 x 2 + (5 – 1)d]
⇒ S5 = 5 [2 + 2d] ………………..(i)
Also, S10 = 10/2 [2 x 2 + (10 – 1)d]
⇒ S10 = 5[4 + 9d] ………………..(ii)
Since S5 = 1/4 (S10 – S5)
So, from eq(i) and (ii), we have:
⇒ 5[2 + 2d] = 1/4 [5(4 + 9d) – 5(2 + 2d)]
⇒ 8 + 8d = 4 + 9d – 2 – 2d
⇒ d = -6
Now we find the 20th term
a20 = a + (20 – 1)d
⇒ a20 = a + 19d
⇒ a20 = 2 + 19(-6) = -112
Hence proved
问题30.如果S 1是AP的(2n +1)项之和,而S 2是其奇数项的和,则证明:S 1 :S 2 =(2n +1):(n +1) )
解决方案:
Let us assume A. P. be a, a + d, a + 2d…
So, S1 = (2n + 1)/2 [2a + (2n + 1 – 1)d]
Now using the formula, we get
⇒ S1 = (2n + 1)/2 [2a + (2n)d]
⇒ S1 = (2n + 1)(a + nd) …………………(i)
Now using the formula, we get
S2 = (n + 1)/2 [2a + (n + 1 – 1) x 2d]
⇒ S2 = (n +1)/2 [2a + 2nd]
⇒ S2 = (n + 1)[a + nd] …………………..(ii)
From eq(i) and (ii), we get :
S1 / S2 = (2n + 1) / (n + 1)
Hence, proved
问题31.找到一个AP,其中任意数量的项之和始终是这些项的平方数的三倍。
解决方案:
Given that Sn = 3n2
So, for n = 1, S1 = 3 x 12 = 3
For n = 2, S2 = 3 x 22 = 12
For n = 3, S3= 3 x 32 = 27 and so on
So, S1 = a1 = 3
a2 = S2 – S1 = 12 – 3 = 9
a3 = S3 – S2 = 27 – 12 = 15 and so on
Hence, the A.P. = 3, 9, 15…
问题32.如果一个AP的n个项之和为nP + 1/2 – n(n – 1)Q,其中P和Q为常数,则求出共同的差。
解决方案:
According to question we have
Sn = nP + 1/2 n(n – 1)Q
For n = 1, S1 = P + 0 = P
For n = 2, S2 = 2P + Q
Also, a1 = S1 = P,
a2 = S2 – S1 = 2P + Q – P = P + Q
Hence, the common difference d = a2 – a1 = P + Q – P = Q
问题33.两个算术级数的n个项之和为5n + 4:9n +6。找到它们的第18个项之比。
解决方案:
Let us considered we have two A.P’s. So, a1 and a2 are the first terms and d1 and d2 is common difference of the A.P’s
According to question we have
(5n + 4) / (9n + 6) = (Sum of n terms in the first A.P.) / (Sum of n terms in the second A.P.)
⇒ (5n + 4) / (9n + 6) = (2a1+ [(n — 1)d1]) / (2a2 + [(n — 1)d2] ………(1)
Now put n = 2 x 18 – 1 = 35 in eq(1), we get
(5 x 35 + 4) / (9 x 35 + 6) = (2a1 + 34d1) / (2a2 + 34d2)
179 /321 = (a1 + 17d1) / (a2 + 17d2) = (18th term of the first A.P.) / (18th term of the second A.P.)
Hence, the ratio of 18th terms = (a1 + 17d1) / (a2 + 17d2) = 179 /321
问题34.两个算术级数的n个项之和为7n + 2:n +4。找到它们的第五项之比。
解决方案:
Let us considered we have two A.P’s. So, a1 and a2 are the first terms and S1 and S2 are the sum of the first n terms.
According to question we have
S1 = n/2 [2a1 + (n – 1)d1]
And, S2 = n/2 [2a2 + (n -1)d2]
Given:
S1 / S2 = (n/2 [2a1 + (n – 1)d1])/(n/2 [2a2 + (n – 1)d2]) = (7n + 2) / (n + 4)
Now we find the ratio of their 5th terms
[2a1 + (9 – 1)d1] / [2a2 + (9 – 1)d2] = (7(9) + 2) / (9 + 4)
[2a1 + (8)d1] / [2a2 + (8)d2] = 65/13
[a1 + (4)d1] / [a2 + (4)d2] = 5/1 = 5 : 1
Hence, the ratio of 5th terms = 5 : 1