Question

If the 3rd and the 9th terms of an arithmetic progession are 4 and -8 respectively, which term of it is zero ?

Solution

Given 3rd term =4 ie  a+2d=4................(1) Also given 9th term =-8 ie a+ 8d=-8............(2) subtracting both equations -6d=12 d=-2 Which gives a=8 let nth term be zero so 0=a+(n-1)d -8=(n-1)(-2) n-1=4 n=5=5th term

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