[질문] 등차수열의합공식이뭐죠?

첫째항이a,공차가d,제n항이m인등차수열의제n항까지의합을Sn이라하면 n(a+m)n {2a+(n-1)d} Sn=n(a+m)n/2 = {2a+(n-1)d}/2