:root {
    --primary-color: #0056b3;
    --secondary-color: #007bff;
    --bg-color: #f8f9fa;
    --card-bg-color: #ffffff;
    --text-color: #343a40;
    --light-text-color: #6c757d;
    --success-color: #28a745;
    --border-radius: 12px;
    --shadow: 0 4px 6px rgba(0, 0, 0, 0.05), 0 1px 3px rgba(0, 0, 0, 0.08);
}

* { margin: 0; padding: 0; box-sizing: border-box; }

body { font-family: 'Poppins', sans-serif; background-color: var(--bg-color); color: var(--text-color); line-height: 1.6; }

.page-header { max-width: 800px; margin: 0 auto; padding: 1.5rem 2rem 0; }
.btn-back { display: inline-block; padding: 0.5rem 1rem; background-color: #e9ecef; color: var(--primary-color); text-decoration: none; border-radius: var(--border-radius); font-weight: 600; transition: background-color 0.3s ease; }
.btn-back:hover { background-color: #dee2e6; }

.main-content { max-width: 800px; margin: 1rem auto 2rem; }
.calculator-header { text-align: center; margin-bottom: 2rem; padding: 0 1rem; }
.calculator-header h1 { color: var(--primary-color); margin-bottom: 0.5rem; }

.calculator-card {
    background-color: var(--card-bg-color);
    border-radius: var(--border-radius);
    box-shadow: var(--shadow);
    border: 1px solid #e9ecef;
    padding: 1.5rem 2rem;
    margin: 0 auto 1.5rem;
}
.calculator-card h3 { font-size: 1.3rem; color: var(--primary-color); margin-bottom: 0.5rem; }
.instruction { font-size: 0.9rem; color: var(--light-text-color); margin-bottom: 1.5rem; }

.form-group { margin-bottom: 1.5rem; }
.form-group label { display: block; margin-bottom: 0.5rem; font-weight: 600; text-align: center; font-size: 0.9rem; }
.form-group input { width: 100%; padding: 0.85rem 0.5rem; border: 1px solid #ced4da; border-radius: var(--border-radius); font-size: 1rem; font-family: 'Poppins', sans-serif; text-align: center;}
.form-group input:focus { outline: none; border-color: var(--secondary-color); box-shadow: 0 0 0 3px rgba(0, 123, 255, 0.25); }
.form-group input:disabled { background-color: #e9ecef; font-weight: bold; color: var(--primary-color); cursor: default; border-style: dashed; }

.radio-group { display: flex; justify-content: center; gap: 1rem; flex-wrap: wrap; background-color: #f8f9fa; padding: 1rem; border-radius: var(--border-radius); margin-bottom: 0; }
.radio-label { display: flex; align-items: center; cursor: pointer; font-size: 1rem; font-weight: 600; }
.radio-label input { margin-right: 0.5rem; width: 1.2em; height: 1.2em; accent-color: var(--primary-color); }
.radio-label span { font-family: monospace, sans-serif; font-size: 1.2rem; }

.equation-row { display: flex; align-items: flex-end; justify-content: center; gap: 0.5rem; flex-wrap: wrap; }
.equation-row .form-group { flex: 1; min-width: 80px; }
.equation-row .symbol { font-size: 1.5rem; font-weight: 700; color: var(--light-text-color); padding-bottom: 0.8rem; }
.equation-row .symbol-squared { font-size: 1.1rem; font-weight: 700; color: var(--light-text-color); padding-bottom: 1.8rem; }

button[type="submit"] { width: 100%; padding: 1rem; background: linear-gradient(90deg, var(--secondary-color) 0%, var(--primary-color) 100%); color: white; border: none; border-radius: var(--border-radius); font-size: 1.1rem; font-weight: 600; cursor: pointer; transition: transform 0.2s ease, box-shadow 0.3s ease; margin-top: 1rem; }
button[type="submit"]:hover { transform: translateY(-2px); box-shadow: 0 4px 10px rgba(0, 86, 179, 0.3); }

.result-display { margin-top: 1.5rem; padding: 1.5rem; background-color: #e7f5ff; border-left: 5px solid var(--success-color); font-size: 1.3rem; font-weight: 600; text-align: center; border-radius: var(--border-radius); word-wrap: break-word; }
.result-display.hidden { display: none; }
.result-display strong { font-size: 2rem; display: block; }

.explanation-article { max-width: 800px; margin: 0 auto 2rem; padding: 2.5rem; background-color: var(--card-bg-color); border-radius: var(--border-radius); box-shadow: var(--shadow); }
.explanation-article h2, .explanation-article h3 { color: var(--primary-color); margin-bottom: 1rem; }
.explanation-article h3 { margin-top: 1.5rem; border-bottom: 1px solid #eee; padding-bottom: 0.5rem; font-size: 1.3rem; }
.explanation-article p, .explanation-article ul { margin-bottom: 1rem; }
.explanation-article ul { padding-left: 25px; }
.explanation-article .formula { text-align: center; font-size: 1.2rem; font-weight: 600; background-color: #f1f3f5; padding: 1rem; border-radius: var(--border-radius); }

.main-footer { text-align: center; padding: 2rem 1rem; color: var(--light-text-color); font-size: 0.9rem; }

@media (max-width: 768px) {
    .main-content { margin-left: 1rem; margin-right: 1rem; }
    .calculator-card { padding: 1.5rem; }
    .radio-group { gap: 0.5rem; justify-content: space-around; }
}