Question

If t4 =22 and t7=37 find Sn and S20 of an A.P

Answer 1

Solution:-

t4 = a4 = a + 3d = 22 ------(1)

t7 = a7 = a + 6d = 37 ------(2)

From eq.(2) - eq. (1)

3d = 15

d = 5

then ,

a = 22 - 3(5)

a = 22 - 15

a = 7

Now ,

Sn = n/2 { 2a + (n - 1)d }

S20 = 20/2 { 2(7) + 19(5) }

= 10 { 14 + 95 }

= 10 × 109

= 1090