Question

find the value of x when 1+6+11+....+x=148​

Answer 1

Answer:

x=36

Step-by-step explanation:

formula is sn=n/2[2a+(n-1)d]

:148=n/2[2(1)+(n-1)5]

296=(2+5n-5)n

296=(-3+5n)n

296=5n^2-3n

5n^2-3n-296=0

factorize

formula is minus b plus or minus rootof b^2-4ac/2a

substitute

a=5

b=-3

c=-296

3 plus or minus root of 9+5920

now root of 5929

=77

3+77/10     or 3-77/10

80/10 since a=5;2a=10        or     -74/10

8

8 th term is the x value

it means 36