A) 15
B) 25
C) 18
D) 10
Correct Answer: A
Solution :
According to the question, we have \[{{a}_{2}}+......+{{a}_{10}}=99\] ?...(i) and \[{{a}_{1}}+.......{{a}_{5}}+{{a}_{7}}+........{{a}_{10}}=89\] ... (ii) Subtracting (ii) from (i), we get \[\Rightarrow \] \[{{a}_{6}}-{{a}_{1}}=10\] \[\Rightarrow \] \[{{a}_{1}}+5d-{{a}_{1}}=10\] \[\Rightarrow \] \[5d=10\,\,\Rightarrow d=2\] Also, \[{{a}_{1}}+{{a}_{5}}=10\,\Rightarrow {{a}_{1}}+{{a}_{1}}+4d=10\] \[\Rightarrow \] \[2{{a}_{1}}+8=10\Rightarrow {{a}_{1}}=1\] \[\therefore \] 8th term \[={{a}_{1}}+7d=1+14=15\]
You need to login to perform this action.
You will be redirected in
3 sec
