1.

The 5th term & 12th term of anArithematic progression are 30 & 65 respectively.Find the sum of its 26th term .​

Answer»

ANSWER:

given,

a5=30

a12=65

s26=?

a5=a+(n-1)d

30=a+(5-1)d

30=a+4d

30-4d=a --------(i)

a12=a+(n-1)d

substituting the value of a from (i),

65=30-4d+(12-1)d

65=30-4d+11d

65=30+7d

65-30=7d

35/7=d

5=d

substituting the value of d in (i)

30-4(5)=a

30-20=a

10=a

s26=n/2[2A+(n-1)d]

=26/2[2×10+(26-1)5]

=13(20+125)

=13×145

=1885



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