Mind tricking maths

[Leah5467]

Hello,this is my question:
why isn't cos(theta)=4/5? My thinking is cosx=adjacent/hypotenuse?

Thank you!

 

[HallsofIvy]

"cosine equals adjacent over hypotenuse for a right triangle. This is not a right triangle- it does not obey the "Pythagorean theorem": \(\displaystyle 4^2+ 5^2= 16+ 25= 41\) not \(\displaystyle 7^2= 49\). Instead, use the "cosine law": in any triangle \(\displaystyle c^2= a^2+ b^2- 2ab cos(C)\) where C is the angle opposite side c.

 

[Harry_the_cat]

It is not a right angled triangle and so does not have an hypotenuse.
Have you learnt the cosine rule?

 

[Leah5467]

ohhh i see thank you!