Question

5. If 6th and 12th term of an A.P. are 13 and 25 respectively, then its 20th term is :
(a) 4
(b) 39
(c) 43
(d) 37
6. How many terms are there in A.P. 1, 3, 5, 7,....
99 ?​

Answer 1

Answer:

5) 41

6) 50

Step-by-step explanation:

5) a6 = 13

a + 5d = 13 (1)

a12 = 25

a + 11d = 25 (2)

Subtracting (1) from (2), we get

a + 11d = 25

a + 5d = 13

- - -

6d = 12

d = 12/6

d = 2

Substituting d = 2 in (1), we get

a + 5(2) = 13

a + 10 = 13

a = 3

a20 = a + 19d

= 3 + 19(2)

= 3 + 38

= 41

6) a = 1

d = 3 - 1 = 2

an = 99

an = a + (n - 1)d

99 = 1 + (n - 1)(2)

98 = (n - 1)(2)

n - 1 = 49

n = 50