Quadratics - Quadratic Formula

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1 9.4 Quadratics - Quadratic Formula Objective: Solve quadratic equations by using the quadratic formula. The general from of a quadratic is ax + bx + c = 0. We will now solve this formula for x by completing the square Example 465. b 4a c a 4a 4a ax + bc +c=0 c c Separate constant from variables Subtract c from both sides ax + b c Divide each term by a a a a x + b a c a b ) ) b = = b a a 4a ) = b 4a 4ac 4a = b 4ac 4a Find the number that completes the square Add to both sides, Get common denominator on right x + b a x + b 4a = b 4a 4ac 4a = b 4ac 4a x + b ) a x + b ) = b 4ac a 4a = ± b 4ac 4a x + b a = ± b 4ac a b ± b 4ac a Factor Solve using the even root property s Subtract b from both sides a This solution is a very important one to us. As we solved a general equation by completing the square, we can use this formula to solve any quadratic equation. Once we identify what a, b, and c are in the quadratic, we can substitute those 343

2 b 4ac values into b ± and we will get our two solutions. This formula is a known as the quadratic fromula Quadratic Formula: if ax + bx+c=0then x= b ± b 4ac a World View Note: Indian mathematician Brahmagupta gave the first explicit formula for solving quadratics in 6. However, at that time mathematics was not done with variables and symbols, so the formula he gave was, To the absolute number multiplied by four times the square, add the square of the middle term; the square root of the same, less the middle term, being divided by twice the square is the value. This would translate to 4ac+ b b a as the solution to the equation ax +bc. We can use the quadratic formula to solve any quadratic, this is shown in the following examples. Example 466. x +3x + =0 3 ± 3 4)) ) 3 ± 9 3 ± 3 ± x= or 4 or a=,b=3,c=, use quadratic formula Evaluate subtraction under root Evaluate root Evaluate ± to get two answers Simplify fractions As we are solving using the quadratic formula, it is important to remember the equation must fist be equal to zero. Example x = 30x + 30x 30x 5x 30x =0 30 ± 30) 45) ) 5) First set equal to zero Subtract 30x and from both sides a=5,b= 30,c=, use quadratic formula 344

3 30 ± ± ± ± 5 5 Evaluate addition inside root Reduce fraction by dividing each term by 0 Example 46. 3x + 4x+=x +6x 5 x 6x+5 x 6x + 5 x x + 3 =0 ± ) 4)3) ) ± 4 5 ± 4 ± 4i 3 ± i 3 First set equation equal to zero Subtractx and 6x and add5 a =, b =, c=3, use quadratic formula Evaluate subtraction inside root Reduce fraction by dividing each term by When we use the quadratic formula we don t necessarily get two unique answers. We can end up with only one solution if the square root simplifies to zero. Example x x +9=0 ± ) 44)9) 4) ± ± 0 ± 0 x= 3 a = 4,b=, c=9, use quadratic formula Evaluate exponents and multiplication Evaluate subtraction inside root Evaluate root Evaluate ± Reduce fraction 345

4 If a term is missing from the quadratic, we can still solve with the quadratic formula, we simply use zero for that term. The order is important, so if the term with x is missing, we have b =0, if the constant term is missing, we have c=0. Example x +7=0 0 ± 0 43)7) 3) ± 4 6 ± i 6 ±i 3 a = 3,b=0missing term),c=7 Evaluate exponnets and multiplication, zeros not needed Reduce, dividing by We have covered three different methods to use to solve a quadratic: factoring, complete the square, and the quadratic formula. It is important to be familiar with all three as each has its advantage to solving quadratics. The following table walks through a suggested process to decide which method would be best to use for solving a problem.. If it can easily factor, solve by factoring. If a= and b is even, complete the square 3. Otherwise, solve by the quadratic formula x 5x + 6=0 x )x 3) =0 or 3 x + 4 ) = = x + x +=5 x +) = 5 x +=± 5 ± 5 x 3x + 4=0 3 ± 3) 4)4) 3 ± i 7 ) The above table is mearly a suggestion for deciding how to solve a quadtratic. Remember completing the square and quadratic formula will always work to solve any quadratic. Factoring only woks if the equation can be factored. 346

5 9.4 Practice - Quadratic Formula Solve each equation with the quadratic formula. ) 4a +6=0 3) x x =0 5) m 3=0 7) 3r r =0 9) 4n 36 = 0 ) v 4v 5= 3) a +3a + 4 =6 5) 3k +3k 4=7 7) 7x + 3x 6 = 9) p + 6p 6 =4 ) 3n +3n = 3 3) x = 7x ) 5x = 7x + 7 7) n = 3n 9) x ) 4a 64 = 0 33) 4p + 5p 36 =3p 35) 5n 3n 5 = 7n 37) 7r = 3r 39) n 9=4 ) 3k +=0 4) 6n =0 6) 5p + p+6=0 ) x x 5 = 0 0) 3b + 6=0 ) x + 4x + = 4) 6n 3n +3= 4 6) 4x 4 = ) 4n +5n = 7 0) m + 4m 4 = 3 ) 3b 3=b 4) 3r + 4= 6r 6) 6a = 5a + 3 ) 6v = 4+6v 30) x = 3) k +6k 6 = k 34) x +x+7=5x +5x 36) 7m 6m + 6= m 3) 3x 3=x 40) 6b = b +7 b 347

Chapter 9, Quadratics from Beginning and Intermediate Algebra by Tyler Wallace is available under a Creative Commons Attribution 3.

Chapter 9, Quadratics from Beginning and Intermediate Algebra by Tyler Wallace is available under a Creative Commons Attribution 3.0 Unported license. 010. 9.1 Quadratics - Solving with Radicals Objective: