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# help

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Let a and b be three-dimensional vectors. Then $$(5a + 7 b) \times (-a+ 3 b) = ka \times b$$
for some scalar k. Find k.

Apr 29, 2020

#1
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The cross-product of a vector with itself is zero, so the LHS reduces to:  15(a x b) - 7(b x a)  =  15(a x b) + 7(a x b) = 22(a x b)

So k = 22

.

Apr 29, 2020