120 - 1+2+......+14+15=?, 195 - 50+43+36+29+22+15=?, 48 - (-7)+(-1)+5+11+17+23=?, 18 - (-12)+(-6)+0+6+12+18=?, 225 - 1+3+5+7+……+29=?,

2下1-2_等差級數

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