Question

In an AP t10=57 and t15=87 then find value of t21.​

Answer 1

$T_{10} = 57 \\a + 9d = 57$       __________[1]

$T_{15} = 87 a + 14d = 87$    _________[2]

From 2 and 1

  a   +  14d =    87

(-)a +(-)  9d =(-) 57

            $5d = 30 \\ d = 6$

$a+9(6)=57 \\a +54=57\\a=57-54 \\a=3$

$T_{21} = ?\\\\a+(n-1)(d)\\3+ 20(6)\\3+120\\123$

hence, ur answer is 123

     

Answer 2

Step-by-step explanation:

put d-6 in eqn 1

a+9×6=57

a=3

put a-3 and. d-6 in eqn 3

t21=3+20×6

t21=123