1.
探索归纳:
\((1)\)如图\(1\),已知\(\triangle ABC\)为直角三角形,\(∠A=90^{\circ}\),若沿图中虚线剪去\(∠A\),则\(∠1+∠2\)等于______
\(A.90^{\circ}\) \(B.135^{\circ}\) \(C.\) \(270^{\circ}\) \(D.315^{\circ}\)
\((2)\)如图\(2\),已知\(\triangle ABC\)中,\(∠A=40^{\circ}\),剪去\(∠A\)后成四边形,则\(∠1+∠2=\)______
\((3)\)如图\(2\),根据\((1)\)与\((2)\)的求解过程,请你归纳猜想\(∠1+∠2\)与\(∠A\)的关系是______
\((4)\)如图\(3\),若把它折成如图\(3\)形状,试探究\(∠1+∠2\)与\(∠A\)的关系并说明理由.