def test_1(): assert triangle(1) == 1
def test_0(): assert triangle(0) == 0
def test_7(): assert triangle(7) == 28
def test_6(): assert triangle(6) == 21
def test_5(): assert triangle(5) == 15
def test_4(): assert triangle(4) == 10
def test_3(): assert triangle(3) == 6
def test_2(): assert triangle(2) == 3