Question

There are 120 page in a novel then sum of all odd and even number is

Answer 1

there are 120 pages in a novel.
1 , 2 , 3 , 4 , 5 , .......... 120
here,
even number are 2, 4, 6, 8, 10, .....120
first term (a ) = 2
common difference (d ) = 2
$T_n=a+(n-1)d\\\\120=2+(n-1)2\\\\n=60$

$S_n=\frac{n}{2}[2a + (n-1)d] =\frac{60}{2}[2\times\:2+(60-1)\times\:2]\\\\=30[122]=3660$

odd number are 1, 3, 5 , 7, 9, ..........119

similarly for odd number ,
$S = \frac{60}{2}[2\times\:1+(60-1)\times\:2]\\\\=30\times\:120=3600$