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If an AP has a=1,tn=20,sn=399 then …

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If an AP has a=1,tn=20,sn=399 then value of n is

    • Posted by Gisha Dominic 5 years, 6 months ago

    CBSE > Class 10 > Mathematics
    • 1 answers

    Tiksha Gupta 5 years, 6 months ago

    20=1+(n-1)d 20-1=(n-1)d 19=(n-1)d--------------------eq1 399=n/2(2+(n-1)d) from eq1 399=n/2(2+19) 399/21=n/2 19=n/2 19/2=n 9.5=n

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