Question

In an A.P. t10=57, t15=87 then find t21​

Answer 1

Answer:

123

Step-by-step explanation:

a + 9d = 57 -1

a + 14d = 87 -2

subtracting 2-1,

5d = 30

d = 6

a = 3

t21 = a + 20d

= 3 + 20×6

= 3 + 120

= 123

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Answer 2

Answer:

t21 = 123

Step-by-step explanation:

t10 = 57

=> t1 + 9d = 57

=> t1 = 57 - 9d ---(1)

t15 = 87

=> t1 + 14d = 87 ---(2)

Substituting eq 1 in eq 2

57 - 9d + 14d = 87

5d = 87 - 57 = 30

d = 6

Substituting d=6 in eq 1

t1 = 57 - (9 × 6) = 57 - 54 = 3

Now, t21 = t1 + 20d = 3 + (20 × 6) = 3 + 120 = 123