Find first negative term 121'117'113'109

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Sia ? 6 years, 5 months ago
Here the given AP is : 121,117,113........
a=121 and d=117-121 = -4
The general term of an AP is given by
an = a+(n-1)d
Let nth term is negative
{tex}\Rightarrow{/tex} a+(n-1)d<0
{tex}\Rightarrow{/tex} 121+(n-1)(-4)<0
{tex} \Rightarrow {/tex} 121-4n+4<0
{tex} \Rightarrow {/tex} -4n+125 < 0
{tex} \Rightarrow {/tex} -4n< -125
Hence, 4n>125
{tex} \Rightarrow {/tex} n >{tex}\frac{{125}}{4}{/tex}
So, n > 31.25
Hence, the 32nd term is first negative term.
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