Question

if in a ap the 11th term is -30 and 21st term is -70 find the 21st term​

Answer 1

Step-by-step explanation:

a+ 10d= - 30........(1)

a + 20 d = -70 .. (2)

-30 -10d +20 d = -70

10d = -40

d = - 4

a= 10

Answer 2

Answer:

Step-by-step explanation:

nth term of AP is given by tₙ = a + (n-1)d

11th term of AP = a + (11 - 1)d = - 30

                        => a + 10d = - 30  -------------------- (1)

21st term of AP = a + (21 - 1)d = - 70

                        => a + 20d = - 70  -------------------- (2)

Subtract (1) from (2)

a + 20d = -70

a + 10d  = -30

-   -            +

-------------------------

10d = - 40

=> d = -4.

Substitute value back in (1),

a + 20(-4) = -70

=> a = -70 + 80 = 10.

now you can find the required term.