Cosine rule question (1 Viewer)

[BlueGas]

I just wanna know why isn't the answer A? Why is it C?

 

[Flop21]

I just wanna know why isn't the answer A? Why is it C?

Because they only give us the angle for ACB, not BAC.

And with the cosine formula the 2 values you enter into the formula, the angle must be between them, with the side your trying to find - on the opposite side.

So take the cosine formula, c^2 = a^2 + b^2 - 2ab * cosC.

c= value opposite the angle (angle = big C), a and b = the values sandwiching the C angle. In this case a and b = d and 25 respectively. And c = 15, and big C = 40 degrees.

So to put it in the formula correctly we use d and 25 as those values outside the angle (ACB).


Does this make sense? Let me know what bits confuse you if you don't understand - I'm finding it a bit hard to explain in words.

 

[BlueGas]

Because they only give us the angle for ACB, not BAC.

And with the cosine formula the 2 values you enter into the formula, the angle must be between them, with the side your trying to find - on the opposite side..

This explains everything, thanks!