In A.P if the 12th term is -13 and the sum of its first four terms is 24.find the sum of its first 10 terms
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it is given that a12= -13and S4=24
now, a+11d= -13--------(i)
we know the formula Sn=n/2(2a+n-1)d
24=4/2(2a+(4-1)d
24=2(2a+3d)
24/2=2a+3d
12=2a+3d---------(ii)
in (i) multiply by 2 then it will become...
2a+ 22d= -26
2a+3d=12
-. -. -
-------------------
19d= -38
d= -2
now, a+11d= -13--------(i)
we know the formula Sn=n/2(2a+n-1)d
24=4/2(2a+(4-1)d
24=2(2a+3d)
24/2=2a+3d
12=2a+3d---------(ii)
in (i) multiply by 2 then it will become...
2a+ 22d= -26
2a+3d=12
-. -. -
-------------------
19d= -38
d= -2
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