Reduce[1/(
4 (\[Delta] - \[Theta]) (-1 + \[Theta]) (\[Delta] + (-2 + \
\[Delta]) \[Theta])) ((
2 (M (-1 + r) (-1 + \[Delta]) - (-2 +
r) (\[Delta] - \[Theta]))^2 (\[Delta] + (-2 + \[Delta]) \
\[Theta]))/(-1 + \[Delta]) +
4 (-1 + r) (M^2 (-1 +
2 r) (-1 + \[Delta]) \[Delta] (-1 + \[Theta]) +
4 M (-1 + \[Delta]) (\[Delta] - \[Theta]) \[Theta] -
2 (\[Delta] - \[Theta]) (\[Delta] + (-2 + \[Delta]) \
\[Theta])) - (
2 (M (-1 + r) (-1 + \[Delta]) - (-2 +
r) (\[Delta] - \[Theta])) (M (-1 + \[Delta]) (-\[Delta] + \
(4 + 4 r (-1 + \[Delta]) - 3 \[Delta]) \[Theta]) -
2 (\[Delta] - \[Theta]) (-2 (-2 +
r) \[Theta] + \[Delta] (-1 +
r + (-3 + r) \[Theta]))))/(-1 + \[Delta])) < 0 &&
0 < r < 1 && 0 < \[Delta] < 1 &&
0 < \[Theta] < 1 && \[Delta] < \[Theta], r]

反正帖子挖上来了你们也不看是吧：
tieba.baidu.com/p/5005284445