Given |u| = 10 at ∠135° and |v| = 5 at ∠30°, what expression can be used to find |u + v|?

The angle between the vectors is 135° − 30° = 105°. For the sum, |u + v|² = 10² + 5² + 2(10)(5)cos(105°). Since cos(105°) = −cos(75°), this is equivalent to 10² + 5² − 2(10)(5)cos(75°), the expression in choice B.

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