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CBSE Class 10 Answered

The fourth term of A.P is twice is 18th term if the 6th term -8 then find the sum of it's first 20 terms  
Asked by Balwinderdhiman631 | 06 Aug, 2018, 05:51: PM
answered-by-expert Expert Answer
begin mathsize 16px style The space fourth space term space of space straight A. straight P space is space twice space is 18 to the power of th space term comma
rightwards double arrow straight a space plus space 3 straight d space equals space 2 left parenthesis straight a space plus space 17 straight d right parenthesis
rightwards double arrow straight a space plus space 3 straight d space equals space 2 straight a space plus space 34 straight d space space
rightwards double arrow straight a space space plus space 31 straight d space equals space 0 space space... left parenthesis 1 right parenthesis
6 to the power of th space term space is space minus 8.
space straight a space plus space 5 straight d space equals space minus 8 space space space.... left parenthesis 2 right parenthesis
Subtract space left parenthesis 2 right parenthesis space from left parenthesis 1 right parenthesis comma space we space get
26 straight d space equals space 8 space
rightwards double arrow space straight d space equals space 4 over 13
Put space straight d space equals space 4 over 13 space in space left parenthesis 1 right parenthesis space comma space straight a space equals space minus 31 space cross times space 4 over 13 space equals space fraction numerator negative 124 over denominator 13 end fraction
Put space the space values space of space straight a space and space straight d space in space the space below space formula comma
straight S subscript 20 space space equals space straight n over 2 left square bracket straight a space plus space left parenthesis straight n space minus 1 right parenthesis straight d right square bracket..... left parenthesis sum space first space 20 space terms right parenthesis



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