Factorising Monic Quadratic Trinomials Minus Minus Form
Transcript
Both of them are negative. In this kind of case again you’re going to consider the signs again. Okay? Very very carefully be very careful. Now this time again I’m going to put my xs is on my left because it doesn’t really change x squared is x times x. I’m going to always start off with that.
Now, look at the factors of 15. Well, negative 15. Actually, because it’s got a negative in front. Now, first of all, guys, we’ve got a negative. What two signs make negative? We could have plus-minus or minus plus isn’t it? Plus minus is negative, negative plus is also negative but we can’t have plus plus or negative negative because those make positive, so we’re going to stick to these signs. Now the factors are 15. It’s 5 and 3 like that, that’s what I meant, okay?
So it could be positive 5 and negative 3 or negative 5 and positive 3 could be one of those two combinations, so you’re gonna think about this very carefully. Remember if you add them, we need to get this middle number. Does positive 3 minus 3 make negative 2?
Well, it makes 2 but it doesn’t make negative 2. What about negative 5 plus 3? Negative 5 plus 3 makes negative 2 doesn’t it? So therefore I’m pretty sure this will be my correct one okay? So write them out on your side if you need to like me. So I’m going to start by putting positive 3 and negative 5. okay positive 3 negative 5. So you draw your cross. It’s going to be 3x whoops I’ll just kind of like rub that off. And x times negative 5 is negative 5x yeah and then add them up 3x minus 5x is negative 2x which is the same as this, so we’ve got the right choice.
So it’s going to be x plus 3 that’s a plus isn’t it, and x minus 5, so x plus 3, x minus 5 is our final answer okay?
So question ten again we have negative negative. So this combination must be plus or minus plus and minus isn’t it? Because plus minus makes negative.So which two combinations.
Well, first of all, I’m going to start off with my x and x because x times x is x squared and 56. 56 the first thing I can think of the top of my head is 7 and 8 and the reason why I chose those is because or 8 minus 7 or 7 minus 8. It becomes negative 1 or 1 isn’t it? So but what do we want? We want negative 1, we want a negative x which means we want negative 1. Through the use of these numbers, how do we make negative one with these numbers?
Well, I have to have the bigger smaller one sorry minus the bigger one to have a negative number. So positive 7 minus 8. Does that make negative 1? Yes, it does. So I’m going to start by using positive 7 and negative 8 okay? And then draw your cross x times 7 is 7x and just make sure that’s a positive 7. x times negative 8 is negative 8x. So 7 minus 8x is negative x. Is that the same? Yes, it is. So therefore these two are the right answers. So x plus 7, x minus 8, okay? Do you get the are you getting quicker?
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