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WBJEE 2015 Math Solved Paper
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© examsnet.com
Question : 27
Total: 80
If y =
e
–
x
cos2x then which of the following differential equations is satisfied ?
d
2
y
d
y
2
+ 2
d
y
d
x
+ 5y = 0
d
2
y
d
y
2
+ 5
d
y
d
x
+ 2y = 0
d
2
y
d
y
2
- 5
d
y
d
x
- 2y = 0
d
2
y
d
y
2
+ 2
d
y
d
x
- 5y = 0
Validate
Solution:
∴ y1 = –y – 2
e
–
x
sin 2x –––– (1)
∴
y
2
= –
y
1
+ 2
e
–
x
sin 2x – 4
e
–
x
cos2x
= –
y
1
– y –
y
1
– 4y
⇒
y
2
+
2
y
1
+ 5y = 0
© examsnet.com
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