find the 20th term if an arithmetic progression 21,16,11,6
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Solution:
a=t1 = 21
t2 =16
t3=11
t4= 6
common difference (d)= t2-t1 = 16-21= -5
d= -5
n=20
According to formula...
tn = a + (n-1)d
t20 = 21+( 20 -1)×(-5)
t20 = 21+(19)×(-5)
t20 = 21+ (-95)
t20 = -74
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