Write the formula of sum an A.P. When first term, last term and number of terms are given. Also find 11th term of A.P. 90, 80, 70, 60........
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Sum of n terms=n/2(a+l)
a_n=a+(n-1)d
a_11=90+(11-1)(-10)
a_11=90+10(-10)
a_11=90-100
a_11= -10
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Sn=n/2(2a+(n-1)d)
a=90
d=80-90=-10
an=a+(n-1)d
a11=90+(11-1)-10
a11=90-100
a11=-10
a=90
d=80-90=-10
an=a+(n-1)d
a11=90+(11-1)-10
a11=90-100
a11=-10
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