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Mathematics

Quadratic equations — ACT practice questions

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What this topic is

Three main methods solve Quadratic equations: factoring, completing the square and the quadratic formula.

Factoring works when the equation splits neatly, as x squared minus five x plus six equals zero becomes x minus two times x minus three equals zero, giving solutions two and three. Completing the square rewrites an expression into vertex form, a skill ACT Mathematics tests by asking which equation is equivalent to a given one.

Another frequent question gives one solution and asks for a missing coefficient, which is found by substituting the known solution. Forgetting the negative square root, and so losing one of the two solutions, is a mistake many wrong choices reflect.

Sample questions

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Question 1Easier

Which of the following gives both zeros of f(x)=2(x−3)2−18?

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Answer: B — 0 and 6

Setting f(x)=0 gives 2((x−3)2−9)=0, so 2x(x−6)=0. Therefore the zeros are 0 and 6.

Question 2Easier

Which of the following is equivalent to x2+10x=11?

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Answer: A — (x+5)2=36

Adding 25 to both sides gives x2+10x+25=36. The left side factors as (x+5)2, so (x+5)2=36.

Question 3Easier

One solution of the equation x2−bx+18=0 is x=3. What is the value of b?

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Answer: C — 9

Substitute x=3: 9−3b+18=0⇒27=3b⇒b=9.

Every question is picked for where you are right now, wrong answers come back until they stick, and it all works offline.

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