the fifth and ninth term of an ap are respectively 35 and 63. Find the ap. Also find the sum of 21 term of ap
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A5=35 and A9=63
A5= a+4d —-(1)
A9=a+8d ——(2)
Solve these we get
-28=-4d
-28/-4=d
7=d
A5=a+4d
35=a+4*7
35-28=a
7=a
Now we have a and d both so
S21=n/2{2a+(n-1)d}
S21=21/2{2*7+(21-1)7}
S21=21/2{14+22*7}
21/2*168
S21=21*84
S21=1764
A5= a+4d —-(1)
A9=a+8d ——(2)
Solve these we get
-28=-4d
-28/-4=d
7=d
A5=a+4d
35=a+4*7
35-28=a
7=a
Now we have a and d both so
S21=n/2{2a+(n-1)d}
S21=21/2{2*7+(21-1)7}
S21=21/2{14+22*7}
21/2*168
S21=21*84
S21=1764
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