Consider the proof. Given: In △ABC, BD ⊥ AC Prove: the formula for the law of cosines, a2 = b2 + c2 – 2bccos(A) Statement Reason 1. In △ABC, BD ⊥ AC 1. given 2. In △ADB, c2 = x2 + h2 2. Pythagorean thm. 3. In △BDC, a2 = (b – x)2 + h2 3. Pythagorean thm. 4. a2 = b2 – 2bx + x2 + h2 4. prop. of multiplication 5. a2 = b2 – 2bx + c2 5. substitution 6. In △ADB, cos(A) = 6. def. cosine 7. ccos(A) = x 7. mult. prop. of equality 8. a2 = b2 – 2bccos(A) + c2 8. ? 9. a2 = b2 + c2 – 2bccos(A) 9. commutative property What is the missing reason in Step 8?

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obenam obenam · Answer 1 · 2018-03-23T11:53:05+01:00

The missing reason is the following:
8. Substitute [tex]x=\cos A[/tex] is the equation of statement 5:
[tex]a^2=b^2-2bcx+c^2[/tex]

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Tiger009 Tiger009 · Answer 2 · 2018-10-08T10:12:27+02:00

Solution: The missing reason in Step 8 is substitution of [tex]x=c\cos (A)[/tex].

Explanation:

The given steps are used to prove the formula for law of cosines.

From step 5 it is noticed that our equation is

[tex]a^2=b^2-2bx+c^2[/tex] ..... (1)

From step 7 it is noticed that the value of [tex]x[/tex] is [tex]c\cos (A)[/tex].

So by substituting [tex]c\cos (A)[/tex] for [tex]x[/tex] in equation (1) we get the equation of step 8, i.e.,

[tex]a^2=b^2-2bc\cos (A)+c^2[/tex]

Hence, the missing reason in Step 8 is substitution of [tex]x=c\cos (A)[/tex].