Find the radian measure of the largest angle of a triangle that whose sides lengths are 8, 9, and 10

the answer is 1.25 radians, but i don't know how to get there
-thanks

We know that the larges angle is opposite the side of length 10, so use law of cosines:

10^2=9^2+8^2-2*9*8*cos(theta)
(100-81-64)/-144=cos(theta), so cos(theta)=5/16, so theta = cos^-1(5/16) = 1.25297262