Home › ZIMSEC O-Level › Mathematics › Equations and inequalities › Solve the equation x squared plus 2x minus 15 eq…
Solve the equation x squared plus 2x minus 15 equals 0.
Ax = 3 or x = -5
Bx = -3 or x = 5
Cx = 5 or x = 3
Dx = -3 or x = -5
Answer & Solution
Correct answer: A. x = 3 or x = -5
1. The equation is already in standard form, with zero on the right hand side.
2. Two numbers are needed whose product is minus 15 and whose sum is plus 2.
3. The product is negative, so one number is positive and the other is negative.
4. Plus 5 and minus 3 multiply to minus 15 and add to plus 2.
5. So the brackets are x plus 5 and x minus 3.
6. A product of two brackets is zero when one bracket is zero.
7. Therefore x equals minus 5 or x equals 3.
8. Answering minus 3 or plus 5 is the sign trap, since the roots are the opposite of the numbers inside the brackets.
_Source: Siyavula Mathematics Grade 11 (Everything Maths, CC BY 4.0), Chapter 2: Equations and inequalities_
Related questions
A repeated expression appears twice in a quadratic equation. What is the useful move?The value under the square root sign in the quadratic formula works out negative. What areBoth sides of an inequality are divided by a negative number. What happens to the sign?Before a quadratic equation is solved by factorisation, what must one side equal?How many solutions can a quadratic equation have at most?