21. Find the number of three-digit natural numbers which are divisible

1.	21. Find the number of three-digit natural numbers which are divisible b2. The nth term of an AP is given by (-4n 15). Find the sum of first 20 terms of this A.P. (-540)
Answer» irst three digit no. which is divisible by 11 is 110 So, a(first term)=110 d(common difference)=11 an(last term)=990 an=a+(n-1)d990=110+(n-1)11 990-110=(n-1)11 880/11=(n-1) 80=n-1 n=81 So the no. of three digits divisible by 11 are 81

21. Find the number of three-digit natural numbers which are divisible b2. The nth term of an AP is given by (-4n 15). Find the sum of first 20 terms of this A.P. (-540)

Answer»

irst three digit no. which is divisible by 11 is 110

So,

a(first term)=110

d(common difference)=11

an(last term)=990

an=a+(n-1)d990=110+(n-1)11

990-110=(n-1)11

880/11=(n-1)

80=n-1

n=81

So the no. of three digits divisible by 11 are 81