Find the sum of the first 25terms of an AP whose nth term is given by an=2-3n.
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an=2-3n
a1=2-3×1
=-1
a2=2-3×2
=-4
D=a2-a1
= -4-(-1)
=-4+1
= -3
Sn= n/2 [2a+(n-1)d]
S25= 25/2[2×-1+(25-1)-3]
=25/2 [-2+24×-3]
= 25/2 [-2-72]
= 25/2 [-74]
=25/2×-74
= 25×-37
= -925
a1=2-3×1
=-1
a2=2-3×2
=-4
D=a2-a1
= -4-(-1)
=-4+1
= -3
Sn= n/2 [2a+(n-1)d]
S25= 25/2[2×-1+(25-1)-3]
=25/2 [-2+24×-3]
= 25/2 [-2-72]
= 25/2 [-74]
=25/2×-74
= 25×-37
= -925
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