I need help on these q's.
-The first term of an arithmetic series is 10 and the 7th term is 46, find the sum of the first 20 terms.
-The 8th term of an arithmetic series is 3 and the ninth term is -1, find the sum of the first 12 terms.
-The fifth term of an arithmetic series is 21 and the 11th term is 45, find the sum of the first 25 terms.
Please help.