作DE‖AB,交BC于点E
则四边形ABED是平行四边形
∴BE=AD=8,CE=17-8=9,∠CED =∠B =55°
∵∠C =70°
∴∠CDE =55°
∴∠CDE=∠CED
∴CD =CE =9
∠B不会是90°的.