  In this paper, we draw constraints on the Cabibbo-Kobayashi-Maskawa (CKM)
angle $\alpha$ and the strong phase $\delta$ from the experimental measurements
of $\ssb$, the CP-violating asymmetries and branching ratios of $B \to \pi^+
\pi^-, \pi^+ \pi^0$ and $K^0 \pi^+$ decays. In the SM, the measured $\ssb$
leads to an upper limit on $\alpha$: $\alpha \leq 180^\circ-\beta$. Taking the
weighted-average of the newest BaBar and Belle measurements of $\spp$ and
$\app$ in $B^0 \to \pi^+\pi^-$ decay as the experimental input, we find the
allowed range of angle $\alpha$, $76^\circ \leq \alpha \leq 135^\circ$ for
$r=|P/T| =0.3 \pm 0.1 $. From the measured CP-averaged branching ratios of $B
\to \pi^+ \pi^-, \pi^+ \pi^0$ and $K^0 \pi^+$ decays, we found an inequality,
$\cos{\alpha} \cos{\delta} \geq 0.45$. The region of $70^\circ \leq \alpha \leq
110^\circ$ is thus excluded by applying this inequality. The combined
constraints on $\alpha$ and $\delta$ are $117^\circ \leq \alpha \leq 135^\circ$
and $-160^\circ \leq \delta \leq -132^\circ$ if we take $\spp=0.49 \pm 0.27$
and $\app=0.51 \pm 0.19$ as the experimental input. The new lower limit on the
angle $\alpha$ is dominated by the measured branching ratios considered in this
paper, and is much stronger than those obtained before.
