Question

Find the sum of all two digit natural no.divisible by 7

Answer 1

Answer:

the two digit numbers divisible by 7 are 14, 28, 42, 56 and 70

therefore the sum of the numbers is

                                                       14

                                                       28

                                                       42

                                                       56

                                                    +  70

                                                     --------

                                                        210

                                                     --------

Answer 2

let the 2 digit no be divisble by 7 be n .

and the no which are divisible by 7 , = 14 , 21 ..........98.

here ,

a (first term ) = 14.

common diff (d) = 7.

last term (l) = 98.

we know the formula , l = a + (n-1) d

by putting those values in that ,

98 = 14 + (n-1)7

98 = 7n+ 7

n = 13.

now, S(n) = n/2 ( a +l)

so , 13/2 ( 14 + 98)

S(n) = 728 ans....