Question

Write the formula of sum an a.P. When first term, last term and number of term are given. Also find 11th term of a.P. 90, 80, 70, 60, ..

Answer 1

Sn=

$\frac{n}{2}(a +l)where \: l \: is \: the \: n \: t h \: term$

t11=a+10d

=90-100=-10

Answer 2

an=n/2(a+an)

11 the term=90+9*-10

90-90=0

11 the term is 0