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Question
Solve the following quadratic equation by completing square method : x2 + 10x + 21 = 0.
Solution
x2 + 10x + 21 = 0
x2 + 10x = -21
To complete the square, find the third term.
`"Third term "=(1/2"coefficient of x")^2`
Coefficient of x=10
`therefore" Third term "=(1/2xx10)^2=(5)^2=25`
Add 25 on both the sides,
x2 + 10x + 25=-21+25
(x + 5)2 = 4
x + 5 = ± 2 ... Taking square root on both sides
x + 5 = 2 or x + 5=-2
∴ x = -3 or x =-7
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