How to find the square of a sum - PSAT Math

Card 0 of 5

Question

Simplify the radical.

$\sqrt{3283}$

Answer

We can break the square root down into 2 roots of 67 and 49. 49 is a perfect square and reduces to 7.

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Question

Expand:

$\left ( 10x+ \frac{1}{5}\right )^{2}$

Answer

Use the perfect square trinomial pattern, setting $A = 10x , B = \frac{1}{5}$ :

$(A + B)^{2} = A^{2} + 2AB + B ^{2}$

$\left ( 10x + \frac{1}{5} \right )^{2} = (10x)^{2} + 2 (10x) \left ( \frac{1}{5} \right ) + \left ( \frac{1}{5} \right ) ^{2}$

$= 100x^{2} + 4x + \frac{1}{25}$

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Question

Expand:

$\left ( 10x+ 0.8\right )^{2}$

Answer

Use the perfect square trinomial pattern, setting :

$(A + B)^{2} = A^{2} + 2AB + B ^{2}$

$(10x + 0.8)^{2} = \left (10x \right )^{2} + 2 \cdot 10x \cdot 0.8 + 0.8^{2}$

$(10x + 0.8)^{2} = 100x^{2} + 16x + 0.64$

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Question

If $\left (5 x^{2 } - x + 1\right )^{2}$ is expanded, what is the coefficient of $x^{2}$ ?

Answer

$\left (5 x^{2 } - x + 1\right )^{2}$

$= \left (5 x^{2 } - x + 1\right ) \left (5 x^{2 } - x + 1\right )$

$= 5 x^{2 } \left (5 x^{2 } - x + 1\right ) - x \left (5 x^{2 } - x + 1\right ) + 1 \left (5 x^{2 } - x + 1\right )$

$= 5 x^{2 } \cdot 5 x^{2 } - 5 x^{2 } \cdot x + 5x^{2 } \cdot 1 - x \cdot 5 x^{2 }+ x \cdot x - x \cdot 1+ 1\cdot 5 x^{2 } - 1 \cdot x + 1\cdot 1$

$= 25 x^{4 } - 5 x^{3 } + 5x^{2 } - 5 x^{3 }+ x ^{2 } - x + 5 x^{2 } - x + 1$

$= 25 x^{4 } - 5 x^{3 } - 5 x^{3 } + 5x^{2 }+ 5 x^{2 } + x ^{2 } - x - x + 1$

$= 25 x^{4 } -10 x^{3 } + 11 x^{2 } -2 x + 1$

The coefficient of $x^{2}$ is therefore 11.

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Question

If $\left (5 x^{2 } - x + 1\right )^{2}$ is expanded, what is the coefficient of $x^{3}$ ?

Answer

$\left (5 x^{2 } - x + 1\right )^{2}$

$= \left (5 x^{2 } - x + 1\right ) \left (5 x^{2 } - x + 1\right )$

$= 5 x^{2 } \left (5 x^{2 } - x + 1\right ) - x \left (5 x^{2 } - x + 1\right ) + 1 \left (5 x^{2 } - x + 1\right )$

$= 5 x^{2 } \cdot 5 x^{2 } - 5 x^{2 } \cdot x + 5x^{2 } \cdot 1 - x \cdot 5 x^{2 }+ x \cdot x - x \cdot 1+ 1\cdot 5 x^{2 } - 1 \cdot x + 1\cdot 1$

$= 25 x^{4 } - 5 x^{3 } + 5x^{2 } - 5 x^{3 }+ x ^{2 } - x + 5 x^{2 } - x + 1$

$= 25 x^{4 } - 5 x^{3 } - 5 x^{3 } + 5x^{2 }+ 5 x^{2 } + x ^{2 } - x - x + 1$

$= 25 x^{4 } -10 x^{3 } + 11 x^{2 } -2 x + 1$

The coefficient of $x^{3}$ is therefore 10.

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