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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. Find the sum of all the terms in the progression.

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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 :
a=-2

Common difference :
d = 5 - (-2)


= 5 + 2


= 7

nth term of an A.P. is


a_n=a+(n-1)d

where, a is first term and d is common difference.


a_n=-2+(n-1)(7)

According to the equation,
a_n>200.


-2+(n-1)(7)>200


(n-1)(7)>200+2


(n-1)(7)>202

Divide both sides by 7.


(n-1)>28.857

Add 1 on both sides.


n>29.857

So, least possible integer value is 30. It means, A.P. has 30 term.

Sum of n terms of an A.P. is


S_n=(n)/(2)[2a+(n-1)d]

Substituting n=30, a=-2 and d=7, we get


S_(30)=(30)/(2)[2(-2)+(30-1)7]


S_(30)=15[-4+(29)7]


S_(30)=15[-4+203]


S_(30)=15(199)


S_(30)=2985

Therefore, the sum of all the terms in the progression is 2985.

User Raksha
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