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CBSE Class 10 Answered

If an = 4 , d= 2 , Sn = -14 . Find n and a.
Asked by chittukala7 | 07 Dec, 2015, 06:39: PM
answered-by-expert Expert Answer

 

begin mathsize 11px style straight a subscript straight n equals 4 space equals space straight a space plus space open parentheses straight n minus 1 close parentheses straight d space equals space straight a space plus space open parentheses straight n minus 1 close parentheses 2 space equals space 4 rightwards double arrow straight a space plus space 2 straight n space equals space 6... space left parenthesis 1 right parenthesis straight S subscript straight n equals straight n over 2 open parentheses straight a plus straight a subscript straight n close parentheses rightwards double arrow straight n over 2 open parentheses straight a plus 4 close parentheses equals negative 14 rightwards double arrow an space plus space 4 straight n space equals space minus space 28... left parenthesis 2 right parenthesis Multiplying space left parenthesis 1 right parenthesis space by space apostrophe straight n apostrophe comma space we space get an space plus space 2 straight n squared space equals space 6 straight n rightwards double arrow an space equals space 6 straight n space minus 2 straight n squared space Substituting space the space above space in space left parenthesis 2 right parenthesis comma space we space get space 6 straight n space minus 2 straight n squared plus 4 straight n space equals space minus 28 rightwards double arrow 2 straight n squared minus 10 straight n minus 28 equals 0 rightwards double arrow straight n squared minus 5 straight n minus 14 equals 0 rightwards double arrow space space straight n equals 7 space or space straight n equals negative 2 Ignoring space the space negative space value comma space straight n space equals space 7 From space left parenthesis 1 right parenthesis comma space straight a space equals space 6 space minus space 2 straight n space equals space 6 space minus space 14 space equals space minus 8 end style

Answered by | 07 Dec, 2015, 07:00: PM

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