两个有向数相乘的法则 /求下列个数的值
(a) (+2)(+7)
(b) (-3)(-9)
(c) (-10)(+7)
(d) (+5)(-3)
(e) (+2)(+13)(+4)
(f) (-3)(-7)(-9)
(g) (-3)(+12)(-5)
(h) (+4)(+17)(-3)