Question

how many terms of theAP: -6,-11/2,-5,......are needed to give the sum-25? Explain with the double answer.

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Answer 1

we know S=n/2{2a + (n-1)d}

a= -6

d= -11/2 -(-6)= 0.5

Putting the values we get

-25 = n/2{2×-6 + (n-1)0.5}

Further solving we get

n^2 - 25n +100=0

n = (-b+√b^2 - 4ac)/2a

n = 20

I hope this will help you!

Happy learning:-)