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Delhi Police Constable Exam Model Paper 6
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© examsnet.com
Question : 87
Total: 100
If a + b = 9 and ab = 20, then find the value of (
a
3
+
b
3
)
.
150
210
189
190
Validate
Solution:
Given:
A + b = 9 and ab = 20
Formula:
a
3
+
b
3
=
(
a
+
b
)
3
–
3
a
b
(
a
+
b
)
Calculation:
a
3
+
b
3
=
(
a
+
b
)
3
–
3
a
b
(
a
+
b
)
⇒
a
3
+
b
3
=
9
3
–
3
×
20
×
9
⇒
a
3
+
b
3
=
729
−
540
⇒
a
3
+
b
3
=
189
Smart Trick
Take a = 5 and b = 4 (a + b = 5 + 4 = 9 and ab = 5 × 4 = 20)
∴
a
3
+
b
3
=
5
3
+
4
3
=
125
+
64
=
189.
© examsnet.com
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