Can someone help me solve for this angle?

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Answer 1 · 2018-03-01T01:17:41

$R=cos^-1(-4/14)$

The first answer is correct.

We'll have to use the Law of Cosines here, which states that

$a^2=b^2-c^2+(2bc)cos(A)$

In our case, $A=R,$ so $a,$ the side opposite to the angle whose cosine we want, is $4.5$

$b$ and $c,$ the other two sides, are $3.5$ and $2$

$4.5^2=2^2+3.5^2-2(2)(3.5)cos(R)$

$20.25=4+12.25-14cos(R)$

$20.25=16.25-14cos(R)$

$4=-14cos(R)$

$cos(R)=(-4/14)$

Recall that if we have $cos(theta)=x, theta=cos^-1(x)$

Therefore,

$R=cos^-1(-4/14)$

The first answer is correct.