OG 22页15题 请求更好解法，请大牛指点

15.The product of all the prime numbers less than 20 is closest to which of the following powers of 10?（C）

（A）10的9次方

（B）10的8次方

（C）10的7次方

（D）10的6次方

（E）10的5次方

个人认为OG上给的方法过于复杂，短时间内不可能做对，求简单思考方法

另有一题，为OG25页39题，看题目无法建立起相应的几何图形，请求大牛们赐教

39.In the xy-plane, if line k has negative slope and passes through the point (-5, r), is the x-intercept of line k positive?(E)

（1）The slope of line k is -5

（2）r>0

请求牛牛们指教

我今天才看到那里，我觉得那题根本没给出解法，我也本来想看看有什么妙解的，结果他就全部乘了而已的

我是这么算的，先把17 19 改写成 18 18 然后分解，最后得出12 11 10 13 566可以看出明显比10的六次方和7次方的中值大

39.In the xy-plane, if line k has negative slope and passes through the point (-5, r), is the x-intercept of line k positive?(E)

（1）The slope of line k is -5

（2）r>0

let the line k=ax+b

since k has negative slope, then a<0

since k passes through (-5, r), then r=-5a+b

need to know: whether b>0 or not

(1) a=-5, then r=25+b, since we do not know r, we cannot determine the sign of b.

(2) r>0, then -5a+b>0, then b>5a, but 5a is a negative number, so we still do not know if b is positive.

(1)+(2) we know r=25+b>0, then b>-25, we still don't know the sign of b.

Therefore (E) is the answer.

谢谢两位大虾！