Two opposite robot-arm problems. One always has exactly one answer. The other usually has several — or none at all.
Every articulated robot arm has to solve two questions that sound almost identical but are computationally opposite. Given the angle of every joint, where does the gripper end up? That's forward kinematics — direct, mechanical, and always solvable. Given where you want the gripper to be, what joint angles get it there? That's inverse kinematics— and it's a fundamentally harder problem, because for most arms there isn't just one answer.