Question

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

Answer 1

Answer:

hope it will help you

Step-by-step explanation:

see the picture

Answer 2

Answer:

Sn = n/2{8 + 5n}

S20 = 1090

Step-by-step explanation:

Given :-

t4 = a + 3d = 22

t7 = a + 6d = 37

Sn = ?

S20 = ?

Subtracting t4 from t7,

a + 6d = 37

- a + 3d = 22

3d = 15

d = 5

Now putting d in t4,

a + 15 = 22

a = 7

We know that

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

= n/2{14 + 5(n - 1)}

= n/2{14 + 5n -5}

= n/2{8 + 5n}

now,

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

S20 = 20/2{14 + 5(19)}

= 10{14 + 95}

= 1090