The nth term of an A.P is given by (-4n+15). Find the sum of first 20 terms of this A.P
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= (-4×1 + 15) + (-4×2 + 15) + ... + (-4×20 + 15)
= -4 (1+2+...+20) + (15×20)
= -4×20×(20+1)/2 + 300
= -840 + 300
= -540
= -4 (1+2+...+20) + (15×20)
= -4×20×(20+1)/2 + 300
= -840 + 300
= -540
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