These are factored by finding the greatest common factor of the given terms. Generally, that can be found by looking at the factors and finding the largest that is common to all terms.
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a) 21 = 7·3; 49 = 7·7. The GCF is 7. Factoring that out gives ...
7(3x +7)
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b) y is the only factor common to both terms.
y(3y -1)
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c) 4 and x^2 are factors common to both terms, assuming the second term is 4x^3.
4x^2(2 +x)
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d) 10 = 2·5; 24 = 2·12; the factors s^3 and s^2 share a factor of s^2; the terms t^2 and t^4 share a factor of t^2.
