Question

how many terms of the ap -10,-7,-4,-1.....must be added to get sum -104?

Answer 1

Use the formula :

Sn=(n/2) x (2a+(n-1)d)

Where Sn= -104, a= -10 & d= -7 +10=3

After substituting the values in the above equation u will get a quadratic equation and then just solve the quadratic equation, u will get two values for 'n' ,one negative and one positive, in that the positive one is right.

Hope it helps you............

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