How many terms of A.P -15,-13,-11... needed to make sum -56
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Answered by
25
given:
a=-15
d=-13-15
=-28
Sn=-56
Sol:
Sn=n/2[2a+(n-1)d)
-56=n/2{2(-15)+(n-1)-28}
-56×2 =n[-30+(n-1)-28]
-112=n[-30-28n+28]
-112=n[-2-28n]
-112=-2n-28n²
28n²+2n-112=0
a=-15
d=-13-15
=-28
Sn=-56
Sol:
Sn=n/2[2a+(n-1)d)
-56=n/2{2(-15)+(n-1)-28}
-56×2 =n[-30+(n-1)-28]
-112=n[-30-28n+28]
-112=n[-2-28n]
-112=-2n-28n²
28n²+2n-112=0
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