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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.​