求大神指教，为什么第一步求出各变量的值后，带到第二个式子出不来G的结果呢，是我写的有问题么
代码如下
Simplify[Solve[{P2g == (m y (-1 - a P1a - P1d + r w1 - r w2 + a y))/(
2 (-2 - 2 m + 2 a y - b y + 2 a m y)),
P1d == (
k (-2 + 2 a y - b y + m (-1 + r w2 + b (P2g - y) + a y)) (4 -
4 a y + 2 b y - m (-4 + 3 a y - 2 b y + y^2)))/(
m (2 m r^2 +
k (-8 + 8 a y - 4 b y + m (-8 + a^2 + 6 a y - 4 b y + y^2)))),
P1a == -((
k (a - y) (-2 + 2 a y - b y +
m (-1 + r w2 + b (P2g - y) + a y)))/(
2 m r^2 +
k (-8 + 8 a y - 4 b y + m (-8 + a^2 + 6 a y - 4 b y + y^2)))),
w1 == (2 r (-2 + 2 a y - b y +
m (-1 + r w2 + b (P2g - y) + a y)))/(
2 m r^2 +
k (-8 + 8 a y - 4 b y +
m (-8 + a^2 + 6 a y - 4 b y + y^2)))}, {P2g, P1d, P1a, w1}]]
G = P2g (m y (-1 - P1d + r w1 - r w2 + a (-P1a + y)) +
P2g (2 - 2 a y + b y + m (2 - 2 a y)))/((1 + m) (-2 + 2 a y -
b y)) - k/2 w2^2