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The sum of first m terms …

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The sum of first m terms of ap is (4m2-m.)If its nth term is 107.find the value of n. Also find the 21st term of this Ap

    • Posted by Pratima Raj 6 years, 3 months ago

    CBSE > Class 10 > Mathematics
    • 1 answers

    Inayat Khan 6 years, 3 months ago

    Putting values of m as 1;2;3... 4m'2-m=4(1)'2-2(1)=2. 4m'2-2m=4(2)'2-2(2)=12. 4m' 2-2m=4(3)'2-2(3)=30. Now;s1=2;s2=12 nd s3=30. Also s1=a1 ;s2-s1=a2. So a1=2 and a2=12-2=10. Therefore d=a2-a1=10-2=8. As ;an=a+(n-1)d. =107=2+(n-1)8. =107=2+8n-8. =112=8n. n=14. Hope you will further solve it and it will help you .

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