Question

Sum of first term and 21st term of an arithemetic sequance is 1000.

a) Find sum of 10th term and 12th term?

b) Find 11

th term?

c) If common difference is 2, then write its algebraic form?

please help me
step by step please ​

Answer 1

$a+a_2_1=1000$

$a + a +20d = 1000\\\\2a +20d=1000$

                            [÷2]

$a+10d=500\\a_1_1 = 500$

$d = 2$

$a+10d=500 \\a+10(2)=500\\a+20=500\\a=500-20\\a=480$

$a=480 \\d=2$

$S_1_0 + S_1_2 =?$

=  $\frac{10}{2} * [2a+9d] + \frac{12}{2} * [2a+11d]$

=  $10a +45d +12a +66d$

=  $22a + 111d$

$=22(480) +111(2)\\=10560 + 222\\=10782$

hope dis helps u