You have a sum,Originally Posted bymargaritas

This is what I like to do since I hate formulas because it is better to understand than to remember the facts. So I am not going to use the sum formula (in fact I do not even know it!) instead rewrite this like this.

You should know that the sum of the first "n" integers is:

Thus,

Thus,

(1)

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The second sum is,

I think it is easier to express it in "sigma notation".

Thus,

Use the fact, the sum of the first "n" integers,

In sigma notation it says,

Thus, in this case,

Note the 50 in front on the "a" and "d" because you add them 50 times.

Simplify your expression,

(2)

Now you have two equation (1) and (2) you should be able to solve for "a" in terms of "T".