13 If y = 3 cos (log x) + 4 sin (log x), show that x2 y2 + xy1 + y = 0
14 If y = Aemx + Benx, show that 
2
2
(
)
0
d y
dy
m
n
mny
dx
dx
−
+
+
=
15 If y = 500e7x + 600e–7x, show that 
2
2
49
d y
y
dx
=
16