Question

if the 6th term of an A.p is 121.find the sum of 1st 11 terms?

Answer 1

$6th\:\:term\:of\:AP\:\:is\:\:121 \\ a_{6} = 121 \\ a + 5d = 121 \\\\ Sum\:\:of\:\:first\:\:11\:\:terms\:\: : \\\\ S_{n} = \frac{n}{2} [2a+(n-1)d] \\\\ S_{11} = \frac{11}{2} [2a + (11-1)d] \\\\ S_{11} = \frac{11}{2} [2a+10d] \\\\ S_{11} = \frac{11}{2} [2(a+5d)] \\\\ S_{11} = 11(a+5d) \\\\ S_{11} = 11(121) \\\\ S_{11} = 1331$