1
Question
Use the formula to expand:
(6) (9 – 7x + 5y)2
(6) (9 – 7x + 5y)2
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Solution
We know that (a + b + c)2 = a2 + b2 + c2 + 2ab + 2bc + 2ac.
Comparing (9 – 7x + 5y)2 with (a + b + c)2 , we get:
a = 9, b = –7x and c = 5y
Substituting these in the above formula:
(9 – 7x + 5y)2 = (9)2 + (–7x)2 + (5y)2 + 2 × 9 × (–7x) + 2 × (–7x) × (5y) + 2 × (5y) × 9
= 81 + 49x2 + 25y2 –126x –70xy + 90y
Comparing (9 – 7x + 5y)2 with (a + b + c)2 , we get:
a = 9, b = –7x and c = 5y
Substituting these in the above formula:
(9 – 7x + 5y)2 = (9)2 + (–7x)2 + (5y)2 + 2 × 9 × (–7x) + 2 × (–7x) × (5y) + 2 × (5y) × 9
= 81 + 49x2 + 25y2 –126x –70xy + 90y
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