Respuesta :
Given:
The first two terms in an arithmetic progression are -2 and 5.
The last term in the progression is the only number in the progression that is greater than 200.
To find:
The sum of all the terms in the progression.
Solution:
We have,
First term : [tex]a=-2[/tex]
Common difference : [tex]d = 5 - (-2)[/tex]
[tex]= 5 + 2[/tex]
[tex]= 7[/tex]
nth term of an A.P. is
[tex]a_n=a+(n-1)d[/tex]
where, a is first term and d is common difference.
[tex]a_n=-2+(n-1)(7)[/tex]
According to the equation, [tex]a_n>200[/tex].
[tex]-2+(n-1)(7)>200[/tex]
[tex](n-1)(7)>200+2[/tex]
[tex](n-1)(7)>202[/tex]
Divide both sides by 7.
[tex](n-1)>28.857[/tex]
Add 1 on both sides.
[tex]n>29.857[/tex]
So, least possible integer value is 30. It means, A.P. has 30 term.
Sum of n terms of an A.P. is
[tex]S_n=\dfrac{n}{2}[2a+(n-1)d][/tex]
Substituting n=30, a=-2 and d=7, we get
[tex]S_{30}=\dfrac{30}{2}[2(-2)+(30-1)7][/tex]
[tex]S_{30}=15[-4+(29)7][/tex]
[tex]S_{30}=15[-4+203][/tex]
[tex]S_{30}=15(199)[/tex]
[tex]S_{30}=2985[/tex]
Therefore, the sum of all the terms in the progression is 2985.
