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Arithmetic Progressions

Question
CBSEENMA10007933

Which term of the A .P. 14, 11, 8..... is - 1? 

Solution

Let nth term of the A.P. 14, 11, 8,.... is – 1.
Here, a = 14, d = 11 – 14 = 8 – 11 = – 3 and an = – 1
because space space space space space space space straight a subscript straight n equals straight a plus left parenthesis straight n minus 1 right parenthesis straight d
rightwards double arrow space space space space space space space minus 1 space equals space 14 space plus space left parenthesis straight n minus 1 right parenthesis left parenthesis negative 3 right parenthesis
rightwards double arrow space space space space space space space space minus 1 equals 14 minus 3 straight n plus 3
rightwards double arrow space space space space space space space 3 straight n minus 1 minus 17 equals 0
rightwards double arrow space space space space space space space space 3 straight n space equals space 18
rightwards double arrow space space space space space space space space straight n space equals 18 over 3 equals 6
Hence, 6th term of the given A .P. is – 1.